reiserfs: destroy allocated commit workqueue

When resirefs is trying to mount a partition, it creates a commit
workqueue (sbi->commit_wq). But when mount fails later, the workqueue
is not freed.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by:  <auxsvr@gmail.com>
Reported-by: Benoît Monin <benoit.monin@gmx.fr>
Cc: Jan Kara <jack@suse.cz>
Cc: stable@vger.kernel.org # >= 3.16
Cc: reiserfs-devel@vger.kernel.org
Fixes: 797d9016
Signed-off-by: Jan Kara <jack@suse.cz>