When returning NFSERR_BADSEQID, the server should not have incremented
the sequence ID, yet the call to nfsrv_checkseqid() a few lines above
does precisely that. Instead, check for a legitimate retransmission
of the original successful request, and reject otherwise, before the
ticket is used and sequence ID incremented.
MFC after: 1 week
Fixes: ff45b9fc1a8c ("Bjorn reported a problem...")
Sponsored by: Klara, Inc.