quota: factor out dquot_write_dquot()

Refactor out dquot_write_dquot() to reduce duplicate code.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230630110822.3881712-2-libaokun1@huawei.com>