rdma_rxe: make rxe work over 802.1q VLAN devices

This patch fixes RDMA/rxe over 802.1q VLAN devices.

Without it, I observed the following behavior:

a) adding a VLAN device to RXE via rxe_net_add() creates a non-functional
   RDMA device. This is caused by the logic in enum_all_gids_of_dev_cb() /
   is_eth_port_of_netdev(), which only considers networks connected to
   "upper devices" of the configured network device, resulting in an empty
   set of gids for a VLAN interface that is an "upper device" itself.
   Later attempts to connect via this rdma device fail in cma_acuire_dev()
   because no gids can be resolved.

b) adding the master device of the VLAN device instead seems to work
   initially, target addresses via VLAN devices are resolved successfully.
   But the connection times out because no 802.1q VLAN headers are
   inserted in the ethernet packets, which are therefore never received.
   This happens because the RXE layer sends the packets via the master
   device rather than the VLAN device.

The problem could be solved by changing either a) or b). My thinking was
that the logic in a) was created deliberately, thus I decided to work on
b). It turns out that the information about the VLAN interface for the gid
at hand is available in the AV information. My patch converts the RXE code
to use this netdev instead of rxe->ndev. With this change, RXE over vlan
works on my test system.

Signed-off-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>