net/mlx5e: RX, Defer page release in legacy rq for better recycling

Currently, fragmented pages from the page pool can be released
in two ways:

1) In the mlx5e driver when trimming off the unused fragments AND the
associated skb fragments have been released. This path allows
recycling of pages to the page pool cache (allow_direct == true).

2) On the skb release path (last fragment release), which
will always release pages to the page pool ring
(allow_direct == false).

Whichever is releasing the last fragment will be decisive on
where the page gets released: the cache or the ring. So we
obviously want to maximize for doing the release from 1.

This patch does that by deferring the release of page fragments
right before requesting new ones from the page pool. A flag is
added to make sure that there's no release before first alloc
and that XDP_TX fragments are not released prematurely.

This is a preparation patch that doesn't unlock the performance
improvements yet. A followup patch will do that.

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