LU-16019 llite: fully disable readahead in kernel I/O path

LU-16019 llite: fully disable readahead in kernel I/O path

In the new kernel (rhel9 or ubuntu 2204), the readahead path may
be out of the control of Lustre CLIO engine:

generic_file_read_iter()
  ->filemap_read()
    ->filemap_get_pages()
      ->page_cache_sync_readahead()
        ->page_cache_sync_ra()

void page_cache_sync_ra()
{
	if (!ractl->ra->ra_pages || blk_cgroup_congested()) {
		if (!ractl->file)
			return;
		req_count = 1;
		do_forced_ra = true;
	}

	/* be dumb */
	if (do_forced_ra) {
		force_page_cache_ra(ractl, req_count);
		return;
	}
	...
}

From the kernel readahead code, even if read-ahead is disabled
(via @ra_pages == 0), it still issues this request as read-ahead
as we will need it to satisfy the requested range. The forced
read-ahead will do the right thing and limit the read to just
the requested range, which we will set to 1 page for this case.

Thus it can not totally avoid the read-ahead in the kernel I/O
path only by setting @ra_pages with 0.
To fully disable the read-ahead in the Linux kernel I/O path, we
still need to set @io_pages to 0, it will set I/O range to 0 in
@force_page_cache_ra():
void force_page_cache_ra()
{
	...
	max_pages = = max_t(unsigned long, bdi->io_pages,
			    ra->ra_pages);
	nr_to_read = min_t(unsigned long, nr_to_read, max_pages);
	while (nr_to_read) {
		...
	}
	...
}

After set bdi->io_pages with 0, it can pass the sanity/101j.

Lustre-change: https://review.whamcloud.com/47993
Lustre-commit: f0cf7fd3cccb2313fa94a307cf862afba256b8d8

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I859a6404abb9116d9acfa03de91e61d3536d3554
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48219
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: xinliang <xinliang.liu@linaro.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>