quota: Split ->set_xstate callback into two

Split ->set_xstate callback into two callbacks - one for turning quotas
on (->quota_enable) and one for turning quotas off (->quota_disable). That
way we don't have to pass quotactl command into the callback which seems
cleaner.

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