Avoid doing multiple route lookups for the same destination IP during forwarding

[eri]

Summary

ip_forward() does a route lookup for testing this packet can be sent to a known destination,
it also can do another route lookup if it detects that an ICMP redirect is needed,
it forgets all of this and handovers to ip_output() to do the same lookup yet again.

This optimisation just does one route lookup during the forwarding path and handovers that to be considered by ip_output().

[ae]

This comment was removed by ae.

[ae]

It looks like you introduced a route reference leak.

[eri]

ae@ i think i spotted the only reference leak i missed.
If not, can you be more verbose?

[ae]

Looks correct to me.

[gnn]

LGTM Approved for commit.

[eri]

Somehow the revision is not closed automatically when setting the editing only to source committers.
Do this manually.