Make the order of directory entries deterministic

[ktock]

Currently, *node.Readdir returns directory entries in undeterministic order
on each call. Though the order of directory entries is up to the filesystem
implementation, undeterministic order on each call results in that the
filesystem clients see random file name on each call of getdents even to the
same seek position in that directory. This commit fixes this issue by sorting
the directory entries by the name.

repro:

package main

import (
	"fmt"
	"os"
	"syscall"
)

func printEntry(f *os.File, position int64) error {
	if _, err := syscall.Seek(int(f.Fd()), position, 0); err != nil {
		return err
	}
	buf := make([]byte, 1024)
	if _, err := syscall.Getdents(int(f.Fd()), buf); err != nil {
		return err
	}
	names := make([]string, 0, 1)
	_, c, newnames := syscall.ParseDirent(buf, 1, names)
	if c < 1 {
		return fmt.Errorf("at least one entry must be parsed")
	}
	fmt.Println(position, ":", newnames[0])
	return nil
}

func main() {
	f, err := os.Open("/usr/local/go/src/runtime")
	if err != nil {
		fmt.Println("Open", err)
		return
	}
	defer f.Close()

	if err := printEntry(f, 10); err != nil {
		panic(err)
	}
	if err := printEntry(f, 10); err != nil {
		panic(err)
	}
}

On the host (expected):

10 : defs_darwin_arm64.go
10 : defs_darwin_arm64.go

In the container (unexpected):

10 : atomic_ppc64x.s
10 : error.go