bcachefs: backpointer_get_key() doesn't pull in btree node

bcachefs: backpointer_get_key() doesn't pull in btree node

We may not need to pull in a btree node when walking backpointers -
don't do so unnecessarily when using backpointer_get_key().

It'll still fall back to backpointer_get_node() in a few situations,
including btree roots (where an iterator can't point at just the key),
and races due to the interior update path not having deleted a
backpointer to an old node yet.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>