IB/IPoIB: Set ah valid flag in multicast send flow

The change of ipoib_ah data structure with adding "valid" flag and
checks of ah->valid in ipoib_start_xmit affected multicast packet flow.

Since the multicast flow doesn't invoke path_rec_start, "ah->valid" flag
remains unset, so that ipoib_start_xmit end up with neigh_refresh_path
instead of sending the packet using neigh.

"ah->valid" has to be set in multicast send flow. As a result IPoIB
starts sending packets via neigh immediately and eliminates 60sec delay
of neigh keep alive interval.

The typical example of this issue are two sequential arpings:

arping 11.134.208.9 -> got response (mcast_send)
arping 11.134.208.9 -> no response (ah->valid = 0)

Fixes: fa9391dbad4b ("RDMA/ipoib: Update paths on CLIENT_REREG/SM_CHANGE events")
Signed-off-by: Denis Drozdov <[e-mail]>
Reviewed-by: Erez Shitrit <[e-mail]>
Reviewed-by: Feras Daoud <[e-mail]>
Signed-off-by: Leon Romanovsky <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/ulp/ipoib/ipoib_multicast.c

···
 			if (neigh && list_empty(&neigh->list)) {
 				kref_get(&mcast->ah->ref);
 				neigh->ah	= mcast->ah;
+				neigh->ah->valid = 1;
 				list_add_tail(&neigh->list, &mcast->neigh_list);
 			}
 		}