squashfs: provide backing_dev_info in order to disable read-ahead

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

We tested the following data by fio.
squashfs image blocksize=128K
test command:

  fio --name basic --bs=? --filename="/mnt/test_file" --rw=? --iodepth=1 --ioengine=psync --runtime=200 --time_based

  turn on squashfs readahead in 5.10 kernel
  bs(k)      read/randread           MB/s
  4            randread              271
  128          randread              231
  1024         randread              246
  4            read                  310
  128          read                  245
  1024         read                  247

  turn off squashfs readahead in 5.10 kernel
  bs(k)      read/randread           MB/s
  4            randread              293
  128          randread              330
  1024         randread              363
  4            read                  338
  128          read                  360
  1024         read                  365

  turn on squashfs readahead and revert the
  commit c1f6925e

("mm: put readahead
  pages in cache earlier") in 5.10 kernel
  bs(k)      read/randread           MB/s
  4           randread               289
  128         randread               306
  1024        randread               335
  4           read                   337
  128         read                   336
  1024        read                   338

Link: https://lkml.kernel.org/r/20211116113141.1391026-1-zhengliang6@huawei.com
Signed-off-by: Zheng Liang <zhengliang6@huawei.com>
Reviewed-by: Phillip Lougher <phillip@squashfs.org.uk>
Cc: Zhang Yi <yi.zhang@huawei.com>
Cc: Hou Tao <houtao1@huawei.com>
Cc: Miao Xie <miaoxie@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>