common/quota: Fix _qmount_options for ext4 journalled quotas

_qmount_options didn't properly replace ext[34] journalled quotas
mount options. As such the mount option string got garbled and the
test (e.g. generic/379) failed.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>