bcachefs: Fix replay_now_at() assert

Journal replay, in the slowpath where we insert keys in journal order,
was inserting keys in the wrong order; keys from early repair come last.

Reported-by:  <syzbot+2c4fcb257ce2b6a29d0e@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>