IB/core: Add netdev to path record

In order to find the sgid_index, one could just query the IB cache
with the correct GID and netdevice. Therefore, instead of storing
the L2 attributes directly in the path, we only store the
ifindex and net and use them later to get the sgid_index.
The vlan_id and smac L2 attributes are removed in a later patch.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-By: Devesh Sharma <devesh.sharma@avagotech.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>