fat: change timestamp updates to use fat_truncate_time

Convert the inode timestamp updates to use fat_truncate_time.

Link: http://lkml.kernel.org/r/2663d3083c4dd62f00b64612c8eaf5542bb05a4c.1538363961.git.sorenson@redhat.com


Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>