locks: pass correct "before" pointer to locks_unlink_lock in generic_add_lease

The argument to locks_unlink_lock can't be just any pointer to a
pointer. It must be a pointer to the fl_next field in the previous
lock in the list.

Cc: <stable@vger.kernel.org> # v3.15+
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/locks.c

@@ -1619,7 +1619,7 @@ static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp
 	smp_mb();
 	error = check_conflicting_open(dentry, arg);
 	if (error)
-		locks_unlink_lock(flp);
+		locks_unlink_lock(before);
 out:
 	if (is_deleg)
 		mutex_unlock(&inode->i_mutex);