btrfs: use next_state instead of rb_next where we can

We can simplify a lot of these functions where we have to cycle through
extent_state's by simply using next_state() instead of rb_next().  In
many spots this allows us to do things like

while (state) {
	/* whatever */
	state = next_state(state);
}

instead of

while (1) {
	state = rb_entry(n, struct extent_state, rb_node);
	n = rb_next(n);
	if (!n)
		break;
}

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