Set the iif for IPv6 packets as well.

This was fixed upstream in a more comprehensive way by 1fb9489b,
but this one-line fix is consistent with the rest of the 3.4
networking code.

Change-Id: I342ca90c6d0213a7aeac9a2b18283530998f81d7
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>