Btrfs: add a missing spin_lock

When fixing up the locking in the delayed ref destruction work I accidently
broke the locking myself ;(.  Add back a spin_lock that should be there and
we are now all set.  Thanks,
Btrfs: add a missing spin_lock

When fixing up the locking in the delayed ref destruction work I accidently
broke the locking myself ;(.  Add back a spin_lock that should be there and
we are now all set.  Thanks,
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/disk-io.c

@@ -3426,6 +3426,7 @@ int btrfs_destroy_delayed_refs(struct btrfs_transaction *trans,
 				mutex_unlock(&head->mutex);
 				btrfs_put_delayed_ref(ref);
 
+				spin_lock(&delayed_refs->lock);
 				continue;
 			}