UPSTREAM: dm: do not allow readahead to limit IO size

Update DM to set the bdi's io_pages.  This fixes reads to be capped at
the device's max request size (even if user's read IO exceeds the
established readahead setting).

Fixes: 9491ae4a ("mm: don't cap request size based on read-ahead setting")
Cc: stable@vger.kernel.org
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit c6d6e9b0f6b4201c77f2cea3964dd122697e3543)
Bug: 120757803
Link: https://patchwork.kernel.org/patch/10746255/
Change-Id: I876f4c5425253c2a0876d5ef7fefe36c4edcb46c