RDMA/cma: Fix handling of IPv6 addressing in cma_use_port

cma_use_port() assumes that the sockaddr is an IPv4 address.  Since
IPv6 addressing is supported (and also to support other address
families) make the code more generic in its address handling.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>