IPoIB: Make sure path is fully initialized before using it

The SA path record query completion can initialize path->pathrec.dlid
before IPoIB's callback runs and initializes path->ah, so we must test
ah rather than dlid.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>