btrfs: make tree_search_for_insert return extent_state

We use this to search for an extent state, or return the nodes we need
to insert a new extent state.  This means we have the following pattern

node = tree_search_for_insert();
if (!node) {
	/* alloc and insert. */
	goto again;
}
state = rb_entry(node, struct extent_state, rb_node);

we don't use the node for anything else.  Making
tree_search_for_insert() return the extent_state means we can drop the
rb_node and clean this up by eliminating the rb_entry.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>