selftests/filesystems: add a vfat RENAME_EXCHANGE test

Add a test for the renameat2 RENAME_EXCHANGE support in vfat, but split it
in a tool that just does the rename exchange and a script that is run by
the kselftests framework on `make TARGETS="filesystems/fat" kselftest`.

That way the script can be easily extended to test other file operations.

The script creates a 1 MiB disk image, that is then formated with a vfat
filesystem and mounted using a loop device.  That way all file operations
are done on an ephemeral filesystem.

Link: https://lkml.kernel.org/r/20220610075721.1182745-5-javierm@redhat.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Alexander Larsson <alexl@redhat.com>
Cc: Christian Kellner <ckellner@redhat.com>
Cc: Chung-Chiang Cheng <cccheng@synology.com>
Cc: Colin Walters <walters@verbum.org>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Peter Jones <pjones@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>