Btrfs: change btrfs_del_leaf to drop locks earlier

btrfs_del_leaf does two things.  First it removes the pointer in the
parent, and then it frees the block that has the leaf.  It has the
parent node locked for both operations.

But, it only needs the parent locked while it is deleting the pointer.
After that it can safely free the block without the parent locked.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.c

@@ -3630,15 +3630,22 @@ noinline int btrfs_del_leaf(struct btrfs_trans_handle *trans,
 {
 	int ret;
 	u64 root_gen = btrfs_header_generation(path->nodes[1]);
+	u64 parent_start = path->nodes[1]->start;
+	u64 parent_owner = btrfs_header_owner(path->nodes[1]);
 
 	ret = del_ptr(trans, root, path, 1, path->slots[1]);
 	if (ret)
 		return ret;
 
+	/*
+	 * btrfs_free_extent is expensive, we want to make sure we
+	 * aren't holding any locks when we call it
+	 */
+	btrfs_unlock_up_safe(path, 0);
+
 	ret = btrfs_free_extent(trans, root, bytenr,
 				btrfs_level_size(root, 0),
-				path->nodes[1]->start,
-				btrfs_header_owner(path->nodes[1]),
+				parent_start, parent_owner,
 				root_gen, 0, 1);
 	return ret;
 }