btrfs: Streamline shared ref check in alloc_reserved_tree_block

Instead of setting "parent" to ref->parent only when dealing with
a shared ref and subsequently performing another check to see
if (parent > 0), check the "node->type" directly and act accordingly.
This makes the code more streamline. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>