ocfs2: free vol_label in ocfs2_delete_osb()

osb->vol_label is malloced in ocfs2_initialize_super but not freed if
error occurs or during umount, thus causing a memory leak.

Signed-off-by: Joseph Qi <joseph.qi@huawei.com>
Reviewed-by: joyce.xue <xuejiufei@huawei.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>