Fix bonding failover in the case both peers failover and the
gratuitous ARP is lost.  In that case, the sender side will create an
ipoib_neigh and issue a path request with the old GID first.  When
skb->dst->neighbour->ha changes due to ARP refresh, this ipoib_neigh
will not be added to the path->list of the path of the new GID,
because the ipoib_neigh already exists.  It will not have an AH
either, because of sender-side failover.  Therefore, it will not get
an AH when the path is resolved.

The solution here is to compare GIDs in ipoib_start_xmit() even if
neigh->ah is invalid.  Comparing with an uninitialized value of
neigh->dgid should be fine, since a spurious match is harmless (and
astronomically unlikely too).

Signed-off-by: Moni Shoua <monis@voltaire.com>
Signed-off-by: Yossi Etigin <yosefe@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>