SUNRPC: Refactor nfsd4_do_encode_secinfo()

Clean up.  This matches a similar API for the client side, and
keeps ULP fingers out the of the GSS mech switch.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Acked-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>