fat: ix mount option parsing

parse_options() is supposed to return value < 0 on error however we
returned 0 (success) in a lot of cases.  This actually was not a problem
in practice because match_token() used by parse_options() is clever and
catches most of the problems for us.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: 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>