FAT: update the second FAT when writing a file

After susccessful write to the FAT partition,
fsck program may print warning message due to different FAT,
provided that the filesystem supports two FATs.

This patch makes the second FAT to be same with the first one
when writing a file.

Signed-off-by: Donggeun Kim <dg77.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>