fat/dir.c: switch to struct __fat_dirent

struct __fat_dirent is what was formerly the kernel struct dirent (that
was different from the userspace struct dirent).

Converting all fat users to struct __fat_dirent will allow us to get rid
of the conflicting struct dirent definition.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
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>