net/smc: Use rdma_read_gid_l2_fields to L2 fields

Use core provided API to fill the source MAC address and use
rdma_read_gid_attr_ndev_rcu() to get stable netdev.

This is preparation patch to allow gid attribute to become NULL when
associated net device is removed.

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