IB/core: Attempt DMAC resolution for only RoCE

Instead of returning 0 (success) for RoCE scenarios where DMAC should
not be resolved, avoid such attempt and make code consistent with
ib_create_user_ah().

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>