nfsd: add support for upcall version 2

nfsd: add support for upcall version 2

Version 2 upcalls will allow the nfsd to include a hash of the kerberos
principal string in the Cld_Create upcall.  If a principal is present in
the svc_cred, then the hash will be included in the Cld_Create upcall.
We attempt to use the svc_cred.cr_raw_principal (which is returned by
gssproxy) first, and then fall back to using the svc_cred.cr_principal
(which is returned by both gssproxy and rpc.svcgssd).  Upon a subsequent
restart, the hash will be returned in the Cld_Gracestart downcall and
stored in the reclaim_str_hashtbl so it can be used when handling
reclaim opens.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>