RDMA/mlx5: Access the prio bypass inside the FDB flow table namespace

Now that we have a specific prio inside the FDB namespace allow retrieving
it from the RDMA side.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>