net/mlx5: fix loopback for Direct Verbs queue

net/mlx5: fix loopback for Direct Verbs queue

In the past, all the queues and other hardware objects were created
through Verbs interface. Currently, most of the objects creation are
migrated to Devx interface by default, including queues. Only when
the DV is disabled by device arg or eswitch is enabled, all or some
of the objects are created through Verbs interface.

When using Devx interface to create queues, the kernel driver
behavior is different from the case using Verbs. The Tx loopback
cannot work properly even if the Tx and Rx queues are configured
with loopback attribute. To fix the support self loopback for Tx, a
Verbs dummy queue pair needs to be created to trigger the kernel to
enable the global loopback capability.

This is only required when TIR is created for Rx and loopback is
needed. Only CQ and QP are needed for this case, no WQ(RQ) needs to
be created.

Bugzilla ID: 645
Fixes: 6deb19e1b2d2 ("net/mlx5: separate Rx queue object creations")
Cc: stable@dpdk.org
Signed-off-by: Bing Zhao <bingz@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>