common/mlx5: fix MPRQ mempool registration

common/mlx5: fix MPRQ mempool registration

Mempool registration code had a wrong assumption that it is always
dealing with packet mempools and always called rte_pktmbuf_priv_flags(),
which returned a random value for different types of mempools.
In particular, it could consider MPRQ mempools as having externally
pinned buffers, which is wrong.
Packet mempools cannot be reliably recognized, but it is sufficient to
check that the mempool is not a packet one, so it cannot have externally
pinned buffers.
Compare mempool private data size to that of packet mempools to check.

Fixes: 690b2a88c2f7 ("common/mlx5: add mempool registration facilities")
Fixes: fec28ca0e3a9 ("net/mlx5: support mempool registration")
Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>