With "bcachefs: Snapshot depth, skiplist fields", we now can't run data
move operations until after bch2_check_snapshots() is complete.

Ideally we'd have the copygc (and rebalance) threads wait until
c->curr_recovery_pass has advanced, but the waitlist handling is tricky
- so for now, move starting copygc back to read_write_late().
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>