btrfs: simplify loop in select_reloc_root()

We have this setup as a loop, but in reality we will never walk back up
the backref tree, if we do then it's a bug.  Get rid of the loop and
handle the case where we have node->new_bytenr set at all.  Previous
check was only if node->new_bytenr != root->node->start, but if it did
then we would hit the WARN_ON() and walk back up the tree.

Instead we want to just return error if ->new_bytenr is set, and then do
the normal updating of the node for the reloc root and carry on.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>