Update GSSv3 use to flattened assertion xdr change

Keep in line with draft-ietf-nfsv4-rpcsec-gssv3-10 changes.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Tom Haynes <loghyr@primarydata.com>
Some idnits

Signed-off-by: Tom Haynes <loghyr@primarydata.com>