Introduce page_pool support in veth driver in order to recycle pages
in veth_convert_skb_to_xdp_buff routine and avoid reallocating the skb
through the page allocator.
The patch has been tested sending tcp traffic to a veth pair where the
remote peer is running a simple xdp program just returning xdp_pass:

veth upstream codebase:
MTU 1500B: ~ 8Gbps
MTU 8000B: ~ 13.9Gbps

veth upstream codebase + pp support:
MTU 1500B: ~ 9.2Gbps
MTU 8000B: ~ 16.2Gbps
Tested-by: Maryam Tahhan <mtahhan@redhat.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>