wgengine/magicsock: fix UDPRelayAllocReq/Resp deadlock (#17831)

Updates #17830

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -2444,7 +2444,10 @@ func (c *Conn) handleDiscoMessage(msg []byte, src epAddr, shouldBeRelayHandshake
 		if !nodeHasCap(c.filt, c.peers.At(peerI), c.self, tailcfg.PeerCapabilityRelay) {
 			return
 		}
-		c.allocRelayEndpointPub.Publish(UDPRelayAllocReq{
+		// [Conn.mu] must not be held while publishing, or [Conn.onUDPRelayAllocResp]
+		// can deadlock as the req sub and resp pub are the same goroutine.
+		// See #17830.
+		go c.allocRelayEndpointPub.Publish(UDPRelayAllocReq{
 			RxFromDiscoKey: sender,
 			RxFromNodeKey:  nodeKey,
 			Message:        req,