SUNRPC: Don't change the RPCSEC_GSS context on a credential that is in use

When a server rejects our credential with an AUTH_REJECTEDCRED or similar,
we need to refresh the credential and then retry the request.
However, we do want to allow any requests that are in flight to finish
executing, so that we can at least attempt to process the replies that
depend on this instance of the credential.

The solution is to ensure that gss_refresh() looks up an entirely new
RPCSEC_GSS credential instead of attempting to create a context for the
existing invalid credential.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>