bcachefs: btree node prefetching in check_topology

btree_and_journal_iter is old code that we want to get rid of, but we're
not ready to yet.

lack of btree node prefetching is, it turns out, a real performance
issue for fsck on spinning rust, so - add it.

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