fat: Fix fat_ent_update_ptr() for FAT12

This fixes the missing update for bhs/nr_bhs in case the caller
accessed from block boundary to first block of boundary.

Signed-off-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>