ofed: allow using IPv6 address in rc_pingpong server

Summary:
The current OFED code allows binding server to IPv6 address. It was added back in https://github.com/linux-rdma/rdma-core/commit/91fc39561d04903cd5b1665d9215a184baa66ba9

Sponsored by: Intel Corporation
MFC after: 3 days

Reviewers: hselasky

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D37196