squashfs: provides backing_dev_info in order to disable read-ahead

hulk inclusion
category: bugfix
bugzilla: 185682 https://gitee.com/openeuler/kernel/issues/I4DDEL



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the commit c1f6925e("mm: put readahead pages in cache earlier")
causes the read performance of squashfs to deteriorate.Through testing,
we find that the performance will be back by closing the readahead of
squashfs. So we want to learn the way of ubifs, provides backing_dev_info
and disable read-ahead.

Signed-off-by: Zheng Liang <zhengliang6@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>

Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>