Fix handling of a NFSv4.1 callback reply from the session cache.

The nfsv4_seqsession() call returns NFSERR_REPLYFROMCACHE when it has a
reply in the session, due to a requestor retry. The code erroneously
assumed a return of 0 for this case. This patch fixes this and adds
a KASSERT(). This would be an extremely rare occurrence. It was found
during code inspection during the pNFS server development.

MFC after: 2 weeks