Previously, a node that was advertising a 4via6 route wouldn't be able
to make use of that same route; the packet would be delivered to
Tailscale, but since we weren't accepting it in handleLocalPackets, the
packet wouldn't be delivered to netstack and would never hit the 4via6
logic. Let's add that support so that usage of 4via6 is consistent
regardless of where the connection is initiated from.
Updates #11304
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ic28dc2e58080d76100d73b93360f4698605af7cb
8f7f9ac17e