btrfs: remove redundant BUG_ON() from __btrfs_inc_extent_ref()

At __btrfs_inc_extent_ref() we are doing a BUG_ON() if we are dealing with
a tree block reference that has a reference count that is different from 1,
but we have already dealt with this case at run_delayed_tree_ref(), making
it useless. So remove the BUG_ON().
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -1514,15 +1514,14 @@ static int __btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
 	btrfs_release_path(path);
 
 	/* now insert the actual backref */
-	if (owner < BTRFS_FIRST_FREE_OBJECTID) {
-		BUG_ON(refs_to_add != 1);
+	if (owner < BTRFS_FIRST_FREE_OBJECTID)
 		ret = insert_tree_block_ref(trans, path, bytenr, parent,
 					    root_objectid);
-	} else {
+	else
 		ret = insert_extent_data_ref(trans, path, bytenr, parent,
 					     root_objectid, owner, offset,
 					     refs_to_add);
-	}
+
 	if (ret)
 		btrfs_abort_transaction(trans, ret);
 out: