fat: fix non-atomic NFS i_pos read

fat_encode_fh() can fetch an invalid i_pos value on systems where 64-bit
accesses are not atomic.  Make it use the same accessor as the rest of the
FAT code.

Signed-off-by: Steven J. Magnani <steve@digidescorp.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>