selftests: net: move xdp_helper to net/lib

Move xdp_helper to net/lib to make it easier for other selftests to use
the helper.

Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20250425071018.36078-2-minhquangbui99@gmail.com


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