net/mlx5e: RX, Enable skb page recycling through the page_pool

net/mlx5e: RX, Enable skb page recycling through the page_pool

Start using the page_pool skb recycling api to recycle all pages back to
the page pool and stop using atomic page reference counting.

The mlx5e driver used to manage in-flight pages using page refcounting:
for each fragment there were 2 atomic write operations happening (one
for building the skb and one on skb release).

The page_pool api introduced a method to track page fragments more
optimally:
* The page's pp_fragment_count is set to a large bias on page alloc
  (1 x atomic write operation).
* The driver tracks the actual page fragments in a non atomic variable.
* When the skb is recycled, pp_fragment_count is decremented
  (atomic write operation).
* When page is released in the driver, the unused number of fragments
  (relative to the bias) is deducted from pp_fragment_count (atomic
  write operation).
* Last page defragmentation will only be an atomic read.

So in total there are `number of fragments + 1` atomic write ops. As
opposed to previously: `2 * frags` atomic writes ops.

Pages are wrapped in a mlx5e_frag_page structure which also contains the
number of fragments. This makes it easy to count the fragments in the
driver.

This change brings performance improvements for the case when the old rx
page_cache had low recycling rates due to head of queue blocking. For a
iperf3 TCP test with a single stream, on a single core (iperf and receive
queue running on same core), the following improvements can be noticed:

* Striding rq:
  - before (net-next baseline): bitrate = 30.1 Gbits/sec
  - after                     : bitrate = 31.4 Gbits/sec (diff: 4.14 %)

* Legacy rq:
  - before (net-next baseline): bitrate = 30.2 Gbits/sec
  - after                     : bitrate = 33.0 Gbits/sec (diff: 8.48 %)

There are 2 temporary performance degradations introduced:

1) TCP streams that had a good recycling rate with the old page_cache
   have a degradation for both striding and linear rq. This is due to
   very low page pool cache recycling: the pages are released during skb
   recycle which will release pages to the page pool ring for safety.
   The following patches in this series will tackle this problem by
   deferring the page release in the driver to increase the
   chance of having pages recycled to the cache.

2) XDP performance is now lower (4-5 %) due to the higher number of
   atomic operations used for fragment management. But this opens the
   door for supporting multiple packets per page in XDP, which will
   bring a big gain.

Otherwise, performance is similar to baseline.

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