btrfs: extract the reference dropping code into it's own helper

This is a big chunk of code in do_walk_down() that will conditionally
remove the reference for the child block we're currently evaluating.
Extract it out into it's own helper and call that from do_walk_down()
instead.

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