net/portmapper: invalidate UPnP mappings asynchronously

invalidateMappingsLocked() makes network requests to invalidate
any active port mappings. Releasing NAT-PMP and PCP mappings never
blocks, but UPnP mappings did block and a slow/non-responding
router could deadlock this code.

Since these invalidations are supposed to be best-effort fire-and-
forget, there is no reason to block on them. Calling Release() in
a separate goroutine means that we can immediately forget the old
mappings and release the lock.

Fixes https://github.com/tailscale/corp/issues/33619

Change-Id: I6d8e53aa104416bc2c63809381f798568353ba7f
Signed-off-by: Francois Marier <francois@tailscale.com>

net/portmapper/portmapper.go

@@ -394,7 +394,7 @@ func (c *Client) listenPacket(ctx context.Context, network, addr string) (nettyp
 func (c *Client) invalidateMappingsLocked(releaseOld bool) {
 	if c.mapping != nil {
 		if releaseOld {
-			c.mapping.Release(context.Background())
+			go c.mapping.Release(context.Background())
 		}
 		c.mapping = nil
 	}