netstack/tcp: fix calculating a number of outstanding packets

In case of GSO, a segment can container more than one packet
and we need to use the pCount() helper to get a number of packets.

PiperOrigin-RevId: 251743020

pkg/tcpip/transport/tcp/snd.go

@@ -779,7 +779,7 @@ func (s *sender) sendData() {
 				break
 			}
 			dataSent = true
-			s.outstanding++
+			s.outstanding += s.pCount(seg)
 			s.writeNext = seg.Next()
 		}
 	}