IB/core: Rename rdma_addr_find_dmac_by_grh

rdma_addr_find_dmac_by_grh resolves dmac, vlan_id and if_index and
downsteram patch will also add hop_limit as an output parameter,
thus we rename it to rdma_addr_find_l2_eth_by_grh.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>