unix: fix fsync for read-only files on aix

On aix, the fsync syscall doesn't work on read-only files. Use
fsync_range instead to allow Fsync to work on any file.

Follow CL 254657 which did the same in package syscall.

For golang/go#41372

Change-Id: Ibc55ecf5c43e27a04281af1a204f847eeafeea63
Reviewed-on: https://go-review.googlesource.com/c/sys/+/362274
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_aix.go

@@ -377,6 +377,11 @@ func (w WaitStatus) TrapCause() int { return -1 }
 
 //sys	fcntl(fd int, cmd int, arg int) (val int, err error)
 
+//sys	fsyncRange(fd int, how int, start int64, length int64) (err error) = fsync_range
+func Fsync(fd int) error {
+	return fsyncRange(fd, O_SYNC, 0, 0)
+}
+
 /*
  * Direct access
  */
@@ -393,7 +398,6 @@ func (w WaitStatus) TrapCause() int { return -1 }
 //sys	Fchmodat(dirfd int, path string, mode uint32, flags int) (err error)
 //sys	Fchownat(dirfd int, path string, uid int, gid int, flags int) (err error)
 //sys	Fdatasync(fd int) (err error)
-//sys	Fsync(fd int) (err error)
 // readdir_r
 //sysnb	Getpgid(pid int) (pgid int, err error)