{net/connstats,wgengine/magicsock}: fix packet counting in connstats

connstats currently increments the packet counter whenever it is called
to store a length of data, however when udp batch sending was introduced
we pass the length for a series of packages, and it is only incremented
ones, making it count wrongly if we are on a platform supporting udp
batches.

Updates tailscale/corp#22075

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -976,7 +976,7 @@ func (de *endpoint) send(buffs [][]byte) error {
 
 		// TODO(raggi): needs updating for accuracy, as in error conditions we may have partial sends.
 		if stats := de.c.stats.Load(); err == nil && stats != nil {
-			stats.UpdateTxPhysical(de.nodeAddr, udpAddr, txBytes)
+			stats.UpdateTxPhysical(de.nodeAddr, udpAddr, len(buffs), txBytes)
 		}
 	}
 	if derpAddr.IsValid() {
@@ -991,7 +991,7 @@ func (de *endpoint) send(buffs [][]byte) error {
 		}
 
 		if stats := de.c.stats.Load(); stats != nil {
-			stats.UpdateTxPhysical(de.nodeAddr, derpAddr, txBytes)
+			stats.UpdateTxPhysical(de.nodeAddr, derpAddr, 1, txBytes)
 		}
 		if allOk {
 			return nil