btrfs: avoid unnecessarily setting the fs to RO and error state at balance_level()

At balance_level(), when trying to promote a child node to a root node, if
we fail to read the child we call btrfs_handle_fs_error(), which turns the
fs to RO mode and sets it to error state as well, causing any ongoing
transaction to abort. This however is not necessary because at that point
we have not made any change yet at balance_level(), so any error reading
the child node does not leaves us in any inconsistent state. Therefore we
can just return the error to the caller.
Reviewed-by: Qu Wenruo <wqu@suse.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/ctree.c

@@ -1040,7 +1040,6 @@ static noinline int balance_level(struct btrfs_trans_handle *trans,
 		child = btrfs_read_node_slot(mid, 0);
 		if (IS_ERR(child)) {
 			ret = PTR_ERR(child);
-			btrfs_handle_fs_error(fs_info, ret, NULL);
 			goto out;
 		}