This unifies the online and offline btree gc passes; we're not yet
running it online.

We now iterate over one level of the btree at a time - the same as
check_extents_to_backpointers(); this ordering preserves order of keys
regardless of btree splits and merges, which will be important when we
re-enable online gc.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>