fat: add a vfat_rename2() and make existing .rename callback a helper

fat: add a vfat_rename2() and make existing .rename callback a helper

Patch series "fat: add support for the renameat2 RENAME_EXCHANGE flag", v6.

The series adds support for the renameat2 system call RENAME_EXCHANGE flag
(which allows to atomically replace two paths) to the vfat filesystem
code.

There are many use cases for this, but we are particularly interested in
making possible for vfat filesystems to be part of OSTree [0] deployments.

Currently OSTree relies on symbolic links to make the deployment updates
an atomic transactional operation.  But RENAME_EXCHANGE could be used [1]
to achieve a similar level of robustness when using a vfat filesystem.

Patch #1 is just a preparatory patch to introduce the RENAME_EXCHANGE
support, patch #2 moves some code blocks in vfat_rename() to a set of
helper functions, that can be reused by tvfat_rename_exchange() that's
added by patch #3 and finally patch #4 adds some kselftests to test it.

This patch (of 4):

Currently vfat only supports the RENAME_NOREPLACE flag which is handled by
the virtual file system layer but doesn't support the RENAME_EXCHANGE
flag.

Add a vfat_rename2() function to be used as the .rename callback and move
the current vfat_rename() handler to a helper.  This is in preparation for
implementing the RENAME_NOREPLACE flag using a different helper function.

Link: https://lkml.kernel.org/r/20220610075721.1182745-1-javierm@redhat.com
Link: https://lkml.kernel.org/r/20220610075721.1182745-2-javierm@redhat.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Christian Kellner <ckellner@redhat.com>
Cc: Peter Jones <pjones@redhat.com>
Cc: Chung-Chiang Cheng <cccheng@synology.com>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Alexander Larsson <alexl@redhat.com>
Cc: Colin Walters <walters@verbum.org>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>