RPC6 requires that it be possible to create endpoints that listen
exclusively for IPv4 or IPv6 connection requests. This is not currently
supported by the RDMA API.

This fixes a server RDMA regression introduced by 37498292 "NFSD:
Create PF_INET6 listener in write_ports".

Signed-off-by: Tom Tucker<tom@opengridcomputing.com>
Tested-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>