btrfs: use rb_find_add() in ulist_rbtree_insert()

Use the rb-tree helper so we don't open code the search and insert
code.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Pan Chuang <panchuang@vivo.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>