selftests: net: add flag to force zerocopy mode in xdp_helper

This commit adds an optional -z flag to xdp_helper. When this flag is
provided, the XDP socket binding is forced to be in zerocopy mode.

Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20250425071018.36078-3-minhquangbui99@gmail.com


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