IB/core: Limit DMAC resolution to userspace QPs

Currently ah_attr is initialized by the ib_cm layer for rdma_cm
based applications. For RoCE transport ah_attr.roce.dmac is already
initialized by ib_cm, rdma_cm either from wc, path record, route
resolve, explicit path record setting depending on active or passive
side QP. Therefore avoid resolving DMAC for QP of kernel consumers.

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>