nfs: make nfs4_cb_sv_ops static

Fixes the following sparse warning:

fs/nfs/callback.c:235:21: warning: symbol 'nfs4_cb_sv_ops' was not
declared. Should it be static?
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/callback.c

@@ -231,12 +231,12 @@ static struct svc_serv_ops nfs41_cb_sv_ops = {
 	.svo_module		= THIS_MODULE,
 };
 
-struct svc_serv_ops *nfs4_cb_sv_ops[] = {
+static struct svc_serv_ops *nfs4_cb_sv_ops[] = {
 	[0] = &nfs40_cb_sv_ops,
 	[1] = &nfs41_cb_sv_ops,
 };
 #else
-struct svc_serv_ops *nfs4_cb_sv_ops[] = {
+static struct svc_serv_ops *nfs4_cb_sv_ops[] = {
 	[0] = &nfs40_cb_sv_ops,
 	[1] = NULL,
 };