ipn/ipnlocal: avoid calling Start from resetForProfileChangeLockedOnEntry

During Shutdown of an ephemeral node, we called Logout (to best effort
delete the node earlier), which then called back into
resetForProfileChangeLockedOnEntry, which then tried to Start
again. That's all a waste of work during shutdown and complicates
other cleanups coming later.

Updates #cleanup

Change-Id: I0b8648cac492fc70fa97c4ebef919bbe352c5d7b
Co-authored-by: Maisem Ali <maisem@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -4930,6 +4930,13 @@ func (b *LocalBackend) initTKALocked() error {
 //
 // b.mu must held on entry. It is released on exit.
 func (b *LocalBackend) resetForProfileChangeLockedOnEntry() error {
+	if b.shutdownCalled {
+		// Prevent a call back to Start during Shutdown, which calls Logout for
+		// ephemeral nodes, which can then call back here. But we're shutting
+		// down, so no need to do any work.
+		b.mu.Unlock()
+		return nil
+	}
 	b.setNetMapLocked(nil) // Reset netmap.
 	// Reset the NetworkMap in the engine
 	b.e.SetNetworkMap(new(netmap.NetworkMap))