ipn/ipnlocal,wgengine/magicsock: re-report NetInfo to new control client (#20025)

magicsock de-duplicates NetInfo callbacks against c.netInfoLast, a cache
that lives on the long-lived magicsock.Conn. That cache survives a control
client swap (interactive login or profile switch), where only the control
client (and its own per-client NetInfo dedup) is replaced. As a result, the
first netcheck after the swap produces a structurally-identical NetInfo
(same PreferredDERP, same NAT shape), magicsock suppresses it as unchanged,
and the new control session never learns our home DERP. Peers can't reach
the node over DERP until some unrelated NetInfo field happens to change.

Add Conn.ResetNetInfoLast to clear the dedup cache, and call it from
LocalBackend.setControlClientLocked whenever a control client is installed,
so the next netcheck re-reports the current NetInfo to the new client.

netInfoLast is only a dedup/optimization cache (all readers nil-guard, and
it is recomputed by every netcheck), so clearing it can only add a delivery,
never lose or misroute one; it is scoped to control-client lifecycle events,
not steady-state operation.

Updates #17887
Fixes #20024

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

ipn/ipnlocal/local.go

@@ -6696,6 +6696,12 @@ func (b *LocalBackend) setControlClientLocked(cc controlclient.Client) {
 	b.cc = cc
 	b.ccAuto, _ = cc.(*controlclient.Auto)
 	b.ignoreControlClientUpdates.Store(cc == nil)
+
+	// magicsock de-dupes NetInfo against a cache that outlives the control
+	// client; clear it on (re)install so the next netcheck re-reports our
+	// NetInfo (notably PreferredDERP, our home DERP) to the new client rather
+	// than suppressing it as unchanged.
+	b.MagicConn().ResetNetInfoLast()
 }
 
 // resetControlClientLocked sets b.cc to nil and returns the old value. If the