Revert "bpf: veth driver panics when xdp prog attached before veth_open"

This reverts commit 5e5dc33d.

This patch fixes the panic maked by 2e0de636. Now Paolo
and Toke suggest reverting the patch 2e0de636 and making
it stronger, so do this first.

Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>