SUNRPC: Add a backpointer from the struct rpc_cred to the rpc_auth

Cleans up an issue whereby rpcsec_gss uses the rpc_clnt->cl_auth. If we want
to be able to add several rpc_auths to a single rpc_clnt, then this abuse
must go.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/net/sunrpc/auth_null.c b/net/sunrpc/auth_null.c
index 3df9fcc..890bd9b 100644
--- a/net/sunrpc/auth_null.c
+++ b/net/sunrpc/auth_null.c
@@ -133,6 +133,7 @@
 
 static
 struct rpc_cred null_cred = {
+	.cr_auth	= &null_auth,
 	.cr_ops		= &null_credops,
 	.cr_count	= ATOMIC_INIT(1),
 	.cr_flags	= RPCAUTH_CRED_UPTODATE,