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>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index e22c5bb..7d7bc8e 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3426,6 +3426,7 @@
 				mutex_unlock(&head->mutex);
 				btrfs_put_delayed_ref(ref);
 
+				spin_lock(&delayed_refs->lock);
 				continue;
 			}