veth: rely on skb_pp_cow_data utility routine

Rely on skb_pp_cow_data utility routine and remove duplicated code.

Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Toke Hoiland-Jorgensen <toke@redhat.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/029cc14cce41cb242ee7efdcf32acc81f1ce4e9f.1707729884.git.lorenzo@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>