bcachefs: Refactor checking of btree topology

bcachefs: Refactor checking of btree topology

Still a lot of work to be done here: we can't yet repair btree topology
issues, but this patch refactors things so that we have better access to
what we need in the topology checks. Next up will be figuring out a way
to do btree updates during gc, before journal replay is done.

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