wgengine,ipn/ipnlocal,tsnet,cmd/tailscaled: remove PeerForIP from the Engine interface

Engine.PeerForIP was pure delegation to the callback that LocalBackend
installs via SetPeerForIPFunc, so external callers going through the
engine were taking a pointless round trip: LocalBackend called
b.e.PeerForIP, which called right back into LocalBackend, and the
netstack UseNetstackForIP hooks in tsnet and tailscaled did the same
dance one layer removed.

Export LocalBackend.PeerForIP and make those callers use it directly.
The engine-internal cold paths (Ping, TSMP disco advertisements,
pendopen diagnostics) still need the lookup and have no netmap of
their own, so SetPeerForIPFunc stays on the interface, but the lookup
method itself is now unexported and gone from the Engine interface.
In tailscaled the UseNetstackForIP hook moves from netstack setup to
just after the LocalBackend is created, since the backend doesn't
exist yet when netstack is wired up.

Updates #12542

Change-Id: Ib1e1a4fa5c84ee0dcb9ce5d1910047f2bab9453c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/netstack.go

@@ -44,11 +44,6 @@ func newNetstack(logf logger.Logf, sys *tsd.System, onlyNetstack bool) (tsd.Nets
 	dialer := sys.Dialer.Get() // must be set by caller already
 
 	if onlyNetstack {
-		e := sys.Engine.Get()
-		dialer.UseNetstackForIP = func(ip netip.Addr) bool {
-			_, ok := e.PeerForIP(ip)
-			return ok
-		}
 		dialer.NetstackDialTCP = func(ctx context.Context, dst netip.AddrPort) (net.Conn, error) {
 			// Note: don't just return ns.DialContextTCP or we'll return
 			// *gonet.TCPConn(nil) instead of a nil interface which trips up