veth: use skb_orphan_partial instead of skb_orphan

As described by commit 9c4c3252 ("skbuff: preserve sock
reference when scrubbing the skb."), orphaning a skb
in the TX path will cause OoO.

Let's use skb_orphan_partial() instead of skb_orphan(), so
that we keep the sk around for queue's selection sake and we
still avoid the problem fixed with commit 4bf9ffa0 ("veth:
Orphan skb before GRO")

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>