ocfs2: Use generic helpers for quotaon and quotaoff

Ocfs2 can just use the generic helpers provided by quota code for
turning quotas on and off when quota files are stored as system inodes.
The only difference is the feature test in ocfs2_quota_on() and that is
covered by dquot_quota_enable() checking whether usage tracking is
enabled (which can happen only if the filesystem has the quota feature
set).

Signed-off-by: Jan Kara <jack@suse.cz>