netfilter: egress: Report interface as outgoing

Otherwise packets in egress chains seem like they are being received by
the interface, not sent out via it.

Fixes: 42df6e1d ("netfilter: Introduce egress hook")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>