archive/tar: fix sparse files support on Darwin

Apple defined the SEEK_HOLE/SEEK_DATA constants in unistd.h
with swapped values, compared to all other UNIX systems.

Fixes #21970

Change-Id: I84a33e0741f0f33a2e04898e96b788b87aa9890f
Reviewed-on: https://go-review.googlesource.com/65570Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/tar/sparse_unix.go

@@ -9,6 +9,7 @@ package tar
 import (
 	"io"
 	"os"
+	"runtime"
 	"syscall"
 )
 
@@ -19,8 +20,12 @@ func init() {
 func sparseDetectUnix(f *os.File) (sph sparseHoles, err error) {
 	// SEEK_DATA and SEEK_HOLE originated from Solaris and support for it
 	// has been added to most of the other major Unix systems.
-	const seekData = 3 // SEEK_DATA from unistd.h
-	const seekHole = 4 // SEEK_HOLE from unistd.h
+	var seekData, seekHole = 3, 4 // SEEK_DATA/SEEK_HOLE from unistd.h
+
+	if runtime.GOOS == "darwin" {
+		// Darwin has the constants swapped, compared to all other UNIX.
+		seekData, seekHole = 4, 3
+	}
 
 	// Check for seekData/seekHole support.
 	// Different OS and FS may differ in the exact errno that is returned when