Add ip_ib_mc_map() to convert IPv4 multicast addresses to IPoIB
hardware addresses.  Also add <linux/if_infiniband.h> so INFINIBAND_ALEN
has a home.

The mapping for multicast addresses is described in
  http://www.ietf.org/internet-drafts/draft-ietf-ipoib-ip-over-infiniband-08.txtSigned-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>