staging: exfat: add exfat filesystem code to staging

staging: exfat: add exfat filesystem code to staging

The exfat code needs a lot of work to get it into "real" shape for
the fs/ part of the kernel, so put it into drivers/staging/ for now so
that it can be worked on by everyone in the community.

The full specification of the filesystem can be found at:
  https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20190828160817.6250-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>