buffer: generate decent code for OutMessage.Reset in Go 1.8 beta 2.

internal/buffer/out_message.go

@@ -55,8 +55,22 @@ func init() {
 // Reset resets m so that it's ready to be used again. Afterward, the contents
 // are solely a zeroed fusekernel.OutHeader struct.
 func (m *OutMessage) Reset() {
-	m.payloadOffset = 0
-	m.header = fusekernel.OutHeader{}
+	// Ideally we'd like to write:
+	//
+	//     m.payloadOffset = 0
+	//     m.header = fusekernel.OutHeader{}
+	//
+	// But Go 1.8 beta 2 generates bad code for this
+	// (https://golang.org/issue/18370). Encourage it to generate the same code
+	// as Go 1.7.4 did.
+	if unsafe.Offsetof(m.payload) != 24 {
+		panic("unexpected OutMessage layout")
+	}
+
+	a := (*[3]uint64)(unsafe.Pointer(m))
+	a[0] = 0
+	a[1] = 0
+	a[2] = 0
 }
 
 // OutHeader returns a pointer to the header at the start of the message.