veth: use napi_skb_cache_get_bulk() instead of xdp_alloc_skb_bulk()

Now that we can bulk-allocate skbs from the NAPI cache, use that
function to do that in veth as well instead of direct allocation from
the kmem caches. veth uses NAPI and GRO, so this is both context-safe
and beneficial.

Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>