unix: correct argument order for SyncFileRange syscall on linux/ppc64{,le}

On linux/ppc64{,le} the SYS_SYNC_FILE_RANGE2 syscall is used to
implement SyncFileRange. This syscall has a different argument order
than SYS_SYNC_FILE_RANGE. Apart from that the implementations of both
syscalls are the same, so use a simple wrapper to invoke the syscall
with the correct argument order.

For context see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=edd5cd4a9424f22b0fa08bef5e299d41befd5622

Fixes golang/go#27485

Change-Id: Idc154eab7b7c521a34de821e1d1a97095e96fed0
Reviewed-on: https://go-review.googlesource.com/133215
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux_ppc64x.go

@@ -44,7 +44,6 @@ package unix
 //sys	Splice(rfd int, roff *int64, wfd int, woff *int64, len int, flags int) (n int64, err error)
 //sys	Stat(path string, stat *Stat_t) (err error)
 //sys	Statfs(path string, buf *Statfs_t) (err error)
-//sys	SyncFileRange(fd int, off int64, n int64, flags int) (err error) = SYS_SYNC_FILE_RANGE2
 //sys	Truncate(path string, length int64) (err error)
 //sys	Ustat(dev int, ubuf *Ustat_t) (err error)
 //sys	accept(s int, rsa *RawSockaddrAny, addrlen *_Socklen) (fd int, err error)
@@ -129,3 +128,11 @@ func Poll(fds []PollFd, timeout int) (n int, err error) {
 	}
 	return poll(&fds[0], len(fds), timeout)
 }
+
+//sys	syncFileRange2(fd int, flags int, off int64, n int64) (err error) = SYS_SYNC_FILE_RANGE2
+
+func SyncFileRange(fd int, off int64, n int64, flags int) error {
+	// The sync_file_range and sync_file_range2 syscalls differ only in the
+	// order of their arguments.
+	return syncFileRange2(fd, flags, off, n)
+}