RDMA/cma: Provide a function to set RoCE path record L2 parameters

Introduce a helper function to set path record L2 fields for RoCE.
This includes setting GID type, destination mac address and netdev
ifindex.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>