The code for handing file overwrite incorrectly calculated the amount of
data to write when writing to the last non-cluster aligned chunk. Fix
this by ensuring that no more data than the 'filesize' is written to disk.
While touching min()-based calculations, change it to type-safe min_t()
function.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>

This patch finally fixes the issue revealed by the test script from the
previous patch. The correctness of the change has been also verified by
the following additional test scripts:

--->8-fat_test2.sh---
#!/bin/bash
make sandbox_defconfig
make
dd if=/dev/zero of=/tmp/10M.img bs=1024 count=10k
mkfs.vfat -v /tmp/10M.img
cat >/tmp/cmds <<EOF
x
host bind 0 /tmp/10M.img
fatls host 0
mw 0x1000000 0x0a434241 0x1000 # "ABC\n"
mw 0x1100000 0x0a464544 0x8000 # "DEF\n"
fatwrite host 0 0x1000000 file0001.raw 0x1000
fatwrite host 0 0x1000000 file0002.raw 0x1000
fatwrite host 0 0x1000000 file0003.raw 0x1000
fatwrite host 0 0x1000000 file0004.raw 0x1000
fatwrite host 0 0x1000000 file0005.raw 0x1000
fatrm host 0 file0002.raw
fatrm host 0 file0004.raw
fatls host 0
fatwrite host 0 0x1100000 file0007.raw 0x2000
fatwrite host 0 0x1100000 file0007.raw 0x1f00
reset
EOF
./u-boot </tmp/cmds
#verify
rm -r /tmp/result /tmp/model
mkdir /tmp/result
mkdir /tmp/model
yes ABC | head -c 4096 >/tmp/model/file0001.raw
yes ABC | head -c 4096 >/tmp/model/file0003.raw
yes ABC | head -c 4096 >/tmp/model/file0005.raw
yes DEF | head -c 7936 >/tmp/model/file0007.raw
mcopy -n -i /tmp/10M.img ::file0001.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0003.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0005.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0007.raw /tmp/result
hd /tmp/10M.img
if diff -urq /tmp/model /tmp/result
then
	echo Test okay
else
	echo Test fail
fi
--->8-fat_test3.sh---
#!/bin/bash
make sandbox_defconfig
make
dd if=/dev/zero of=/tmp/10M.img bs=1024 count=10k
mkfs.vfat -v /tmp/10M.img
cat >/tmp/cmds <<EOF
x
host bind 0 /tmp/10M.img
fatls host 0
mw 0x1000000 0x0a434241 0x1000 # "ABC\n"
mw 0x1100000 0x0a464544 0x8000 # "DEF\n"
fatwrite host 0 0x1000000 file0001.raw 0x1000
fatwrite host 0 0x1000000 file0002.raw 0x1000
fatwrite host 0 0x1000000 file0003.raw 0x1000
fatwrite host 0 0x1000000 file0004.raw 0x1000
fatwrite host 0 0x1000000 file0005.raw 0x1000
fatrm host 0 file0002.raw
fatrm host 0 file0004.raw
fatls host 0
fatwrite host 0 0x1100000 file0007.raw 0x2000
fatwrite host 0 0x1100000 file0007.raw 0x2100
reset
EOF
./u-boot </tmp/cmds
#verify
rm -r /tmp/result /tmp/model
mkdir /tmp/result
mkdir /tmp/model
yes ABC | head -c 4096 >/tmp/model/file0001.raw
yes ABC | head -c 4096 >/tmp/model/file0003.raw
yes ABC | head -c 4096 >/tmp/model/file0005.raw
yes DEF | head -c 8448 >/tmp/model/file0007.raw
mcopy -n -i /tmp/10M.img ::file0001.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0003.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0005.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0007.raw /tmp/result
hd /tmp/10M.img
if diff -urq /tmp/model /tmp/result
then
	echo Test okay
else
	echo Test fail
fi
--->8-fat_test4.sh---
#!/bin/bash
make sandbox_defconfig
make
dd if=/dev/zero of=/tmp/10M.img bs=1024 count=10k
mkfs.vfat -v /tmp/10M.img
cat >/tmp/cmds <<EOF
x
host bind 0 /tmp/10M.img
fatls host 0
mw 0x1000000 0x0a434241 0x1000 # "ABC\n"
mw 0x1100000 0x0a464544 0x8000 # "DEF\n"
mw 0x1200000 0x0a494847 0x8000 # "GHI\n"
fatwrite host 0 0x1000000 file0001.raw 0x1000
fatwrite host 0 0x1000000 file0002.raw 0x1000
fatwrite host 0 0x1000000 file0003.raw 0x1000
fatwrite host 0 0x1000000 file0004.raw 0x1000
fatwrite host 0 0x1000000 file0005.raw 0x1000
fatrm host 0 file0002.raw
fatrm host 0 file0004.raw
fatls host 0
fatwrite host 0 0x1100000 file0007.raw 0x900
fatwrite host 0 0x1200000 file0007.raw 0x900 0x900
fatwrite host 0 0x1100000 file0007.raw 0x900 0x1200
fatwrite host 0 0x1200000 file0007.raw 0x900 0x1b00
reset
EOF
./u-boot </tmp/cmds
#verify
rm -r /tmp/result /tmp/model
mkdir /tmp/result
mkdir /tmp/model
yes ABC | head -c 4096 >/tmp/model/file0001.raw
yes ABC | head -c 4096 >/tmp/model/file0003.raw
yes ABC | head -c 4096 >/tmp/model/file0005.raw
yes DEF | head -c 2304 >/tmp/model/file0007.raw
yes GHI | head -c 2304 >>/tmp/model/file0007.raw
yes DEF | head -c 2304 >>/tmp/model/file0007.raw
yes GHI | head -c 2304 >>/tmp/model/file0007.raw
mcopy -n -i /tmp/10M.img ::file0001.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0003.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0005.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0007.raw /tmp/result
hd /tmp/10M.img
if diff -urq /tmp/model /tmp/result
then
	echo Test okay
else
	echo Test fail
fi
--->8---
Feel free to prepare a proper sandbox/py_test based tests based on
the provided test scripts.