nfsv3: introduce nfs3_set_ds_client

The flexfiles layout wants to create DS connection over NFSv3.
Add nfs3_set_ds_client to allow that to happen.

Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Tom Haynes <Thomas.Haynes@primarydata.com>

include/linux/nfs_fs_sb.h

@@ -77,10 +77,6 @@ struct nfs_client {
 	/* Client owner identifier */
 	const char *		cl_owner_id;
 
-	/* Our own IP address, as a null-terminated string.
-	 * This is used to generate the mv0 callback address.
-	 */
-	char			cl_ipaddr[48];
 	u32			cl_cb_ident;	/* v4.0 callback identifier */
 	const struct nfs4_minor_version_ops *cl_mvops;
 	unsigned long		cl_mig_gen;
@@ -108,6 +104,11 @@ struct nfs_client {
 #define NFS_SP4_MACH_CRED_COMMIT   6	/* COMMIT */
 #endif /* CONFIG_NFS_V4 */
 
+	/* Our own IP address, as a null-terminated string.
+	 * This is used to generate the mv0 callback address.
+	 */
+	char			cl_ipaddr[48];
+
 #ifdef CONFIG_NFS_FSCACHE
 	struct fscache_cookie	*fscache;	/* client index cache cookie */
 #endif