Revert "ext4: Allow parallel DIO reads"

euler inclusion
category: perf
bugzilla: 12801
CVE: NA
---------------------------

This reverts commit 16c54688.

This patch will lead to the mixreadwrite performance degradation,
just revert it.

Signed-off-by: yangerkun <yangerkun@huawei.com>
Reviewed-by: Miao Xie <miaoxie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>