fs/affs/super.c: destroy sbi mutex in affs_kill_sb()

Call mutex_destroy() on superblock mutex in affs_kill_sb() otherwise mutex
debugging code isn't able to detect that mutex is used after being freed.
(thanks to Jan Kara for complete definition).

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>