bcachefs: Fix bogus verify_replicas_entry() assert

verify_replicas_entry() is only for newly created replicas entries -
existing entries on disk may have unknown data types, and we have real
verifiers for them.

Reported-by:  <syzbot+73414091bd382684ee2b@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>