xen-netfront: introduce rx page recyling

xen-netfront: introduce rx page recyling

Recycling pages lets us avoid the page allocator when possible, as
similar approach followed by ixgbe and mlx{4,5} drivers. Introduce
a small buffer pool tracking outstanding pages. We increase page
refcount by 1 to avoid stack freeing the page in upper layers. Recycling
of pages is then possible on inflight skbs, by the time we process N
requests by the stack and thus when allocating new Rx requests we
attempting at reusing the oldest page in the pool if and only if
page._refcount is 1. Otherwise we just decrement the refcount (on
free_page).

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Orabug: 26107942