nfsd4: const-ify nfs_cb_version4

Signed-off-by: NChristoph Hellwig <hch@lst.de>

fs/nfsd/nfs4callback.c

@@ -706,7 +706,7 @@ static const struct rpc_procinfo nfs4_cb_procedures[] = {
 };
 
 static unsigned int nfs4_cb_counts[ARRAY_SIZE(nfs4_cb_procedures)];
-static struct rpc_version nfs_cb_version4 = {
+static const struct rpc_version nfs_cb_version4 = {
 /*
  * Note on the callback rpc program version number: despite language in rfc
  * 5661 section 18.36.3 requiring servers to use 4 in this field, the