bpf: Allow to specify ifindex for skb in bpf_prog_test_run_skb

Now skb->dev is unconditionally set to the loopback device in current net
namespace. But if we want to test bpf program which contains code branch
based on ifindex condition (eg filters out localhost packets) it is useful
to allow specifying of ifindex from userspace. This patch adds such option
through ctx_in (__sk_buff) parameter.

Signed-off-by: Dmitry Yakunin <zeil@yandex-team.ru>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200803090545.82046-3-zeil@yandex-team.ru