NFS: Set EXCHGID4_FLAG_SUPP_MOVED_MIGR

Broadly speaking, v4.1 migration is untested.  There are no servers
in the wild that support NFSv4.1 migration.  However, as server
implementations become available, we do want to enable testing by
developers, while leaving it disabled for environments for which
broken migration support would be an unpleasant surprise.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/Kconfig

@@ -140,6 +140,17 @@ config NFS_V4_1_IMPLEMENTATION_ID_DOMAIN
 	  If the NFS client is unchanged from the upstream kernel, this
 	  option should be set to the default "kernel.org".
 
+config NFS_V4_1_MIGRATION
+	bool "NFSv4.1 client support for migration"
+	depends on NFS_V4_1
+	default n
+	help
+	  This option makes the NFS client advertise to NFSv4.1 servers that
+          it can support NFSv4 migration.
+
+          The NFSv4.1 pieces of the Linux NFSv4 migration implementation are
+          still experimental.  If you are not an NFSv4 developer, say N here.
+
 config NFS_V4_SECURITY_LABEL
 	bool
 	depends on NFS_V4_2 && SECURITY

fs/nfs/nfs4proc.c

@@ -6596,8 +6596,14 @@ static int _nfs4_proc_exchange_id(struct nfs_client *clp, struct rpc_cred *cred,
 	struct nfs41_exchange_id_args args = {
 		.verifier = &verifier,
 		.client = clp,
+#ifdef CONFIG_NFS_V4_1_MIGRATION
 		.flags = EXCHGID4_FLAG_SUPP_MOVED_REFER |
-			EXCHGID4_FLAG_BIND_PRINC_STATEID,
+			 EXCHGID4_FLAG_BIND_PRINC_STATEID |
+			 EXCHGID4_FLAG_SUPP_MOVED_MIGR,
+#else
+		.flags = EXCHGID4_FLAG_SUPP_MOVED_REFER |
+			 EXCHGID4_FLAG_BIND_PRINC_STATEID,
+#endif
 	};
 	struct nfs41_exchange_id_res res = {
 		0