s4:torture: Fix samba3.smb2.name-mangling on btrfs

s4:torture: Fix samba3.smb2.name-mangling on btrfs

If a file is removed from or added to the directory after the most recent call
to opendir() or rewinddir(), whether a subsequent call to readdir() returns
an entry for that file is unspecified."
https://pubs.opengroup.org/onlinepubs/009604599/functions/readdir.html

As it is unspecified, the different filesystems on Linux implement this
differently:

ext4:

./a.out
opendir(foo)
creat(foo/bar)
readdir() loop
  readdir entry: bar
  readdir entry: ..
  readdir entry: .
readdir() detected the newly created file `foo`

btrfs:

./a.out
opendir(foo)
creat(foo/bar)
readdir() loop
  readdir entry: .
  readdir entry: ..
readdir() did not detect the newly created file `foo`

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>