btrfs: unify logic to decide if we need to walk down into a node during snapshot delete

We currently duplicate the logic for walking into a node during snapshot
delete.  In one case it is during the actual delete, and in the other we
use it for deciding if we should reada the block or not.

Factor this code into it's own helper and comment fully what we're
doing, and then update the two users to use the new helper.

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