pNFS/flexfiles: Make local symbol layoutreturn_ops static

Fixes the following sparse warning:

fs/nfs/flexfilelayout/flexfilelayout.c:2114:34: warning:
 symbol 'layoutreturn_ops' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/flexfilelayout/flexfilelayout.c

@@ -2076,7 +2076,7 @@ ff_layout_free_layoutreturn(struct nfs4_xdr_opaque_data *args)
 	kfree(ff_args);
 }
 
-const struct nfs4_xdr_opaque_ops layoutreturn_ops = {
+static const struct nfs4_xdr_opaque_ops layoutreturn_ops = {
 	.encode = ff_layout_encode_layoutreturn,
 	.free = ff_layout_free_layoutreturn,
 };