net/smc: replace function pointer get_netdev()

SMC should not open code the function pointer get_netdev of the
IB device. Replacing ib_query_gid(..., NULL) with
ib_query_gid(..., gid_attr) allows access to the netdev.

Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Suggested-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>