svcrdma: RDMA support not yet compatible with RPC6

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 37498292a "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>