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>