don't check refcount in packet buffer's destroy function

The function is only called when refcount is zero,
so we don't have to check again.

PiperOrigin-RevId: 438171100

pkg/tcpip/stack/packet_buffer.go

@@ -188,9 +188,7 @@ func NewPacketBuffer(opts PacketBufferOptions) *PacketBuffer {
 // pool.
 func (pk *PacketBuffer) DecRef() {
 	pk.packetBufferRefs.DecRef(func() {
-		if pk.packetBufferRefs.refCount == 0 {
-			pkPool.Put(pk)
-		}
+		pkPool.Put(pk)
 	})
 }