quota: factor out dquot_write_dquot()

[ Upstream commit 024128477809f8073d870307c8157b8826ebfd08 ]

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>
Stable-dep-of: dabc8b207566 ("quota: fix dqput() to follow the guarantees dquot_srcu should provide")
Signed-off-by: Sasha Levin <sashal@kernel.org>