switch to ASM vector checksum

overlay/checksum/checksum_fallback.go

@@ -0,0 +1,10 @@
+//go:build !amd64 && !arm64
+
+package checksum
+
+import gvisorchecksum "gvisor.dev/gvisor/pkg/tcpip/checksum"
+
+// Checksum delegates to gvisor on architectures without a hand-written body.
+func Checksum(buf []byte, initial uint16) uint16 {
+	return gvisorchecksum.Checksum(buf, initial)
+}