net/mlx5: improve vectorized MPRQ descriptors locality

net/mlx5: improve vectorized MPRQ descriptors locality

There is a performance penalty for the replenish scheme
used in vectorized Rx burst for both MPRQ and SPRQ.
Mbuf elements are being filled at the end of the mbufs
array and being replenished at the beginning. That leads
to an increase in cache misses and the performance drop.
The more Rx descriptors are used the worse the situation.

Change the allocation scheme for vectorized MPRQ Rx burst:
allocate new mbufs only when consumed mbufs are almost
depleted (always have one burst gap between allocated and
consumed indices). Keeping a small number of mbufs allocated
improves cache locality and improves performance a lot.

Unfortunately, this approach cannot be applied to SPRQ Rx
burst routine. In MPRQ Rx burst we simply copy packets from
external MPRQ buffers or attach these buffers to mbufs.
In SPRQ Rx burst we allow the NIC to fill mbufs for us.
Hence keeping a small number of allocated mbufs will limit
NIC ability to fill as many buffers as possible. This fact
offsets the advantage of better cache locality.

Fixes: 0f20acbf5eda ("net/mlx5: implement vectorized MPRQ burst")

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