{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/magicsock.go

@@ -1523,7 +1523,7 @@ func (c *Conn) receiveIP(b []byte, ipp netip.AddrPort, cache *ippEndpointCache)
 	ep.lastRecvUDPAny.StoreAtomic(now)
 	ep.noteRecvActivity(ipp, now)
 	if stats := c.stats.Load(); stats != nil {
-		stats.UpdateRxPhysical(ep.nodeAddr, ipp, len(b))
+		stats.UpdateRxPhysical(ep.nodeAddr, ipp, 1, len(b))
 	}
 	return ep, true
 }