udf: destroy sbi mutex in put_super

Call mutex_destroy() on superblock mutex in udf_put_super()
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>
Signed-off-by: Jan Kara <jack@suse.cz>