fat: fix unaligned errors

A couple of buffers in the fat code are declared as an array of bytes.
But it is then cast up to a structure with 16bit and 32bit members.
Since GCC assumes structure alignment here, we have to force the
buffers to be aligned according to the structure usage.

Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>