xprtrdma: Add rdma6 option to support NFS/RDMA IPv6

RFC 5666: The "rdma" netid is to be used when IPv4 addressing
is employed by the underlying transport, and "rdma6" for IPv6
addressing.

Add mount -o proto=rdma6 option to support NFS/RDMA IPv6 addressing.

Changes from v2:
 - Integrated comments from Chuck Level, Anna Schumaker, Trodt Myklebust
 - Add a little more to the patch description to describe NFS/RDMA
   IPv6 suggested by Chuck Level and Anna Schumaker
 - Removed duplicated rdma6 define
 - Remove Opt_xprt_rdma mountfamily since it doesn't support

Signed-off-by: Shirley Ma <shirley.ma@oracle.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/super.c

@@ -191,6 +191,7 @@ static const match_table_t nfs_mount_option_tokens = {
 
 enum {
 	Opt_xprt_udp, Opt_xprt_udp6, Opt_xprt_tcp, Opt_xprt_tcp6, Opt_xprt_rdma,
+	Opt_xprt_rdma6,
 
 	Opt_xprt_err
 };
@@ -201,6 +202,7 @@ static const match_table_t nfs_xprt_protocol_tokens = {
 	{ Opt_xprt_tcp, "tcp" },
 	{ Opt_xprt_tcp6, "tcp6" },
 	{ Opt_xprt_rdma, "rdma" },
+	{ Opt_xprt_rdma6, "rdma6" },
 
 	{ Opt_xprt_err, NULL }
 };
@@ -1456,6 +1458,8 @@ static int nfs_parse_mount_options(char *raw,
 				mnt->flags |= NFS_MOUNT_TCP;
 				mnt->nfs_server.protocol = XPRT_TRANSPORT_TCP;
 				break;
+			case Opt_xprt_rdma6:
+				protofamily = AF_INET6;
 			case Opt_xprt_rdma:
 				/* vector side protocols to TCP */
 				mnt->flags |= NFS_MOUNT_TCP;