staging: exfat: remove fs_func struct.

Remove 'fs_func struct' and change indirect calls to direct calls.

The following issues are described in exfat's TODO.
> Create helper function for exfat_set_entry_time () and
> exfat_set_entry_type () because it's sort of ugly to be calling the same functionn directly and other code calling through  the fs_func struc ponters ...

The fs_func struct was used for switching the helper functions of fat16/fat32/exfat.
Now, it has lost the role of switching, just making the code less readable.

Signed-off-by: Tetsuhiro Kohada <Kohada.Tetsuhiro@dc.MitsubishiElectric.co.jp>
Link: https://lore.kernel.org/r/20200123102445.123033-1-Kohada.Tetsuhiro@dc.MitsubishiElectric.co.jp


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