ViaRoutesForPeer runs for every peer of every map response. Before it
looks at Via, it converts every ACL to grants and resolves each grant's
sources and destinations against the whole node set. Only via grants
can add to the result and ACLs never carry via, so without a via grant
all of that work was discarded. On large tailnets it dominated map
generation. Return early instead.
BenchmarkViaRoutesForPeer, mean per peer (Apple M3 Pro):
policy nodes before after
global 1000 21.8us 9ns, 0 allocs
self 1000 45.0us 9ns, 0 allocs
via 1000 8.1us unchanged
via-mixed 1000 69.0us unchanged
Fixes#3512
Jonas Schwartz
f8018f177a