IB/cma: Set default gid type to RoCEv2

RoCEv2 is the preferred RDMA protocol for Ethernet link layer because
of its advantages over RoCEv1. For better user experience make it the
default choice for RDMA_CM connections if device/port support it.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>