btrfs: make tree search for insert more generic and use it for tree_search

btrfs: make tree search for insert more generic and use it for tree_search

With a slight extension of tree_search_for_insert (fill the return node
and parent return parameters) we can avoid calling __etree_search from
tree_search, that could be removed eventually in followup patches.

Signed-off-by: David Sterba <dsterba@suse.com>