bcachefs: Convert gc to new accounting

Rewrite fsck/gc for the new accounting scheme.

This adds a second set of in-memory accounting counters for gc to use;
like with other parts of gc we run all trigger in TRIGGER_GC mode, then
compare what we calculated to existing in-memory accounting at the end.

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