Curently, we are using 32 bit multiplication to calculate the offset,
so the result will always be 32 bit.
This can silently cause file system corruption when performing a write
operation on partition larger than 4 GiB.

This patch address the issue by simply promoting the terms to 64 bit,
and let compilers decide how to do the multiplication efficiently.

Signed-off-by: Ma Haijun <mahaijuns@gmail.com>