IB/core: Add missing check for addr_resolve callback return value

When calling rdma_resolve_ip inside rdma_addr_find_l2_eth_by_grh,
the return status of the request was ignored in the callback function
causing a successful return and an empty dmac.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Alex Vesker <valex@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>