sunrpc: fix code that makes auth_gss send destroy_cred message (try #2)

sunrpc: fix code that makes auth_gss send destroy_cred message (try #2)

There's a bit of a chicken and egg problem when it comes to destroying
auth_gss credentials. When we destroy the last instance of a GSSAPI RPC
credential, we should send a NULL RPC call with a GSS procedure of
RPCSEC_GSS_DESTROY to hint to the server that it can destroy those
creds.

This isn't happening because we're setting clearing the uptodate bit on
the credentials and then setting the operations to the gss_nullops. When
we go to do the RPC call, we try to refresh the creds. That fails with
-EACCES and the call fails.

Fix this by not clearing the UPTODATE bit for the credentials and adding
a new crdestroy op for gss_nullops that just tears down the cred without
trying to destroy the context.

The only difference between this patch and the first one is the removal
of some minor formatting deltas.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>