SUNRPC: Add a helper rpcauth_lookup_generic_cred()

The NFSv4 protocol allows clients to negotiate security protocols on the
fly in the case where an administrator on the server changes the export
settings and/or in the case where we may have a filesystem migration event.

Instead of having the NFS client code cache credentials that are tied to a
particular AUTH method it is therefore preferable to have a generic credential
that can be converted into whatever AUTH is in use by the RPC client when
the read/write/sillyrename/... is put on the wire.

We do this by means of the new "generic" credential, which basically just
caches the minimal information that is needed to look up an RPCSEC_GSS,
AUTH_SYS, or AUTH_NULL credential.

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

fs/nfs/unlink.c

@@ -234,7 +234,7 @@ nfs_async_unlink(struct inode *dir, struct dentry *dentry)
 	if (data == NULL)
 		goto out;
 
-	data->cred = rpcauth_lookupcred(NFS_CLIENT(dir)->cl_auth, 0);
+	data->cred = rpc_lookup_cred();
 	if (IS_ERR(data->cred)) {
 		status = PTR_ERR(data->cred);
 		goto out_free;