IB/mlx5: Add GSI QP wrapper

mlx5 creates special GSI QPs that has limited ability to control the P_Key
of transmitted packets. The sent P_Key is taken from the QP object,
similarly to what happens with regular UD QPs.

Create a software wrapper around GSI QPs that with the following patches
will be able to emulate the functionality of a GSI QP including control of
the P_Key per work request.

Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>