exfat: add out of tree module

exfat: add out of tree module

>From an email conversation with the person responsible for upstreaming
the exFAT driver, it seems the staging one in kernel 5.4 is not so
good. Excerpts below.

Namjae Jeon:
Hm... exfat in 5.4 kernel  that we did crap shit long time ago is
contributed by someone who we don't know.
This version is unstable and low quality code. We have been improving
it continuously.
and staging version exfat is removed from linux 5.7 kernel.

linux exfat oot  version is a backport of exfat in linux 5.7 kernel to
support lower version kernel, and it is a real.
You can see the patch history fro linux-exfat-oot.
this version support timezone and boot sector verification feature newly.
and better filesystem structure and much clean code quality that
reviewed by high profile kernel developers. and add many bug fixes.
And this version is officially maintained by me and kernel guys.

I would not recommend to use staging exfat version.

Signed-off-by: Rosen Penev <rosenp@gmail.com>