nfsd4: fix discarded security labels on setattr

Security labels in setattr calls are currently ignored because we forget
to set label->len.

Cc: [e-mail]
Reported-by: Jeff Layton <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs4xdr.c

···
 		label->data = kzalloc(dummy32 + 1, GFP_KERNEL);
 		if (!label->data)
 			return nfserr_jukebox;
+		label->len = dummy32;
 		defer_free(argp, kfree, label->data);
 		memcpy(label->data, buf, dummy32);
 	}