bpf: fix virtio-net's length calc for XDP_PASS

In commit 6870de43 ("bpf: make virtio compatible w/
bpf_xdp_adjust_tail") i didn't account for vi->hdr_len during new
packet's length calculation after bpf_prog_run in receive_mergeable.
because of this all packets, if they were passed to the kernel,
were truncated by 12 bytes.

Fixes:6870de43

 ("bpf: make virtio compatible w/ bpf_xdp_adjust_tail")
Reported-by: David Ahern <dsahern@gmail.com>

Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>