fat: add functions to update and truncate timestamps appropriately

Add the fat-specific inode_operation ->update_time() and
fat_truncate_time() function to truncate the inode timestamps from 1
nanosecond to the appropriate granularity.

Link: http://lkml.kernel.org/r/38af1ba3c3cf0d7381ce7b63077ef8af75901532.1538363961.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>