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: