net/mlx5: handle MPRQ incompatibility with external buffers

net/mlx5: handle MPRQ incompatibility with external buffers

Multi-Packet Rx queue uses PMD-managed buffers to store packets.
These buffers are externally attached to user mbufs.
This conflicts with the feature that allows using user-managed
externally attached buffers in an application.
Fall back to SPRQ in case external buffers mempool is configured.
The limitation is already documented in mlx5 guide.

Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>