bcachefs: Check btree topology at startup

bcachefs: Check btree topology at startup

When initial btree gc was changed to overlay journal keys as it walks
the btree, it also stopped checking btree topology.

Previously, checking btree topology was a fairly complicated affair -
but it's much easier now that btree_ptr_v2 has min_key in the pointer.

This rewrites the old range_checks code and uses it in both runtime and
initial gc.

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