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>