NFS: Add nfs4_unique_id boot parameter

An optional boot parameter is introduced to allow client
administrators to specify a string that the Linux NFS client can
insert into its nfs_client_id4 id string, to make it both more
globally unique, and to ensure that it doesn't change even if the
client's nodename changes.

If this boot parameter is not specified, the client's nodename is
used, as before.

Client installation procedures can create a unique string (typically,
a UUID) which remains unchanged during the lifetime of that client
instance.  This works just like creating a UUID for the label of the
system's root and boot volumes.

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

fs/nfs/nfs4_fs.h

@@ -380,6 +380,9 @@ extern bool nfs4_disable_idmapping;
 extern unsigned short max_session_slots;
 extern unsigned short send_implementation_id;
 
+#define NFS4_CLIENT_ID_UNIQ_LEN		(64)
+extern char nfs4_client_id_uniquifier[NFS4_CLIENT_ID_UNIQ_LEN];
+
 /* nfs4sysctl.c */
 #ifdef CONFIG_SYSCTL
 int nfs4_register_sysctl(void);