ext4: Use generic helpers for quotaon and quotaoff

Ext4 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 ext4_quota_on_sysfile() but the same
is achieved in dquot_quota_enable() by checking whether usage tracking
for the corresponding quota type is enabled (which can happen only if
quota feature is set).

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>