use to identify if the socket is the same one that a cached request
came in on. It is set by nfsrvd_addsock() to a unique value generated
by incrementing an unsigned 64bit static variable for each assignment
and then the value of xp_sockref is tested to see if it is equal to
the value that was saved with the cached reply.
Submitted by: rmacklem
Reviewed by: dfr
Approved by: kib (mentor)
201e7488b6