net/mlx5e: RX, Break the wqe bulk refill in smaller chunks

To avoid overflowing the page pool's cache, don't release the
whole bulk which is usually larger than the cache refill size.
Group release+alloc instead into cache refill units that
allow releasing to the cache and then allocating from the cache.

A refill_unit variable is added as a iteration unit over the
wqe_bulk when doing release+alloc.

For a single ring, single core, default MTU (1500) TCP stream
test the number of pages allocated from the cache directly
(rx_pp_recycle_cached) increases from 0% to 52%:

+---------------------------------------------+
| Page Pool stats (/sec)  |  Before |   After |
+-------------------------+---------+---------+
|rx_pp_alloc_fast         | 2145422 | 2193802 |
|rx_pp_alloc_slow         |       2 |       0 |
|rx_pp_alloc_empty        |       2 |       0 |
|rx_pp_alloc_refill       |   34059 |   16634 |
|rx_pp_alloc_waive        |       0 |       0 |
|rx_pp_recycle_cached     |       0 | 1145818 |
|rx_pp_recycle_cache_full |       0 |       0 |
|rx_pp_recycle_ring       | 2179361 | 1064616 |
|rx_pp_recycle_ring_full  |     121 |       0 |
+---------------------------------------------+

With this patch, the performance for legacy rq for the above test is
back to baseline.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>