Fix new_seqno()'s recovery from seqno mismatch

Fix new_seqno()'s recovery from seqno mismatch

A sequence number mismatch occurs when an invalid copy (with a stale
sequence number) gets written.  To continue after the error, we need
to use the current sequence number.  But new_seqno() used the larger
one of the two, which is correct only as long as sequence numbers do
not overflow.  Overflow is rather unlikely, though.