RDMA: Introduce and use GID attr helper to read RoCE L2 fields

Instead of RoCE drivers figuring out vlan, smac fields while working on
QP/AH, provide a helper routine to read the L2 fields such as vlan_id and
source mac address.

This moves logic from mlx5 driver to core for wider usage for RoCE ports.

This is a preparation patch to allow detaching netdev in subsequent patch.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>