IPoIB: Fix ipoib_hard_header() return value

If you have a patched up dhcp server (and dhclient), they will use
AF_PACKET/SOCK_DGRAM pair to send dhcp packets over IPoIB.

However, when testing an upstream kernel, this has been broken for a
very long time (I tested 2.6.34, 2.6.38, 3.0, 3.1, 3.8, HEAD).

It turns out that the hard_header routine in ipoib is not following
the API and is returning 0 even when it pushed data onto the skb.
This then causes af_packet.c to overwrite the header just pushed with
data from user space.

Fixing this gets DHCP working on IPoIB.

Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/ulp/ipoib/ipoib_main.c

@@ -828,7 +828,7 @@ static int ipoib_hard_header(struct sk_buff *skb,
 	 */
 	memcpy(cb->hwaddr, daddr, INFINIBAND_ALEN);
 
-	return 0;
+	return sizeof *header;
 }
 
 static void ipoib_set_mcast_list(struct net_device *dev)