net/mlx5: implement vectorized MPRQ burst

net/mlx5: implement vectorized MPRQ burst

MPRQ (Multi-Packet Rx Queue) processes one packet at a time using
simple scalar instructions. MPRQ works by posting a single large buffer
(consisted of multiple fixed-size strides) in order to receive multiple
packets at once on this buffer. A Rx packet is then copied to a
user-provided mbuf or PMD attaches the Rx packet to the mbuf by the
pointer to an external buffer.

There is an opportunity to speed up the packet receiving by processing
4 packets simultaneously using SIMD (single instruction, multiple data)
extensions. Allocate mbufs in batches for every MPRQ buffer and process
the packets in groups of 4 until all the strides are exhausted. Then
switch to another MPRQ buffer and repeat the process over again.

The vectorized MPRQ burst routine is engaged automatically in case
the mprq_en=1 devarg is specified and the vectorization is not disabled
explicitly by providing rx_vec_en=0 devarg. There is a limitation:
LRO is not supported and scalar MPRQ is selected if it is on.

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