xprtrdma: Support IPv6 in xprt_rdma_set_port

Clean up a harmless oversight. xprtrdma's ->set_port method has
never properly supported IPv6.

This issue has never been a problem because NFS/RDMA mounts have
always required "port=20049", thus so far, rpcbind is not invoked
for these mounts.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>