fat: factor out reusable code in vfat_rename() as helper functions

The vfat_rename() function is quite big and there are code blocks that can
be moved into helper functions.  This not only simplify the implementation
of that function but also allows these helpers to be reused.

For example, the helpers can be used by the handler of the RENAME_EXCHANGE
flag once this is implemented in a subsequent patch.

Link: https://lkml.kernel.org/r/20220610075721.1182745-3-javierm@redhat.com


Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.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: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Peter Jones <pjones@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>