fat: allow time_offset to be up to 24 hours

Currently we limit values of time_offset mount option to be between -12
and 12 hours.  However e.g.  zone GMT+12 can have a DST correction on top
which makes the total time difference 13 hours.  Update the checks in
mount option parsing to allow offset of upto 24 hours to allow for unusual
cases.

Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Volker Kuhlmann <list0570@paradise.net.nz>
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>