staging: exfat: explain the fs_sync() issue in TODO

We've seen several incorrect patches for fs_sync() calls in the exfat driver.
Add code to the TODO that explains this isn't just a delete code and refactor,
but that actual analysis of when the filesystem should be flushed to disk
needs to be done.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>

Link: https://lore.kernel.org/r/9837.1570042895@turing-police


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>