ext4: trivial quota cleanup

The patch is aimed to reorganize and simplify quota code a bit.
Quota code is itself complex enough, but we can make it more readable
in some places:
- Move quota option parsing to separate functions.
- Simplify old-quota and journaled-quota mix check.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>