bdi cleanups v6

Hi Jens,

this series contains a bunch of different BDI cleanups.  The biggest item
is to isolate block drivers from the BDI in preparation of changing the
lifetime of the block device BDI in a follow up series.

Changes since v5:
 - improve a commit message
 - improve the stable_writes deprecation printk
 - drop "drbd: remove RB_CONGESTED_REMOTE"
 - drop a few hunks that add a local variable in a otherwise unchanged
   file due to changes in the previous revisions
 - keep updating ->io_pages in queue_max_sectors_store
 - set an optimal I/O size in aoe
 - inherit the optimal I/O size in bcache

Changes since v4:
 - add a back a prematurely removed assignment in dm-table.c
 - pick up a few reviews from Johannes that got lost

Changes since v3:
 - rebased on the lasted block tree, which has some of the prep
   changes merged
 - extend the ->ra_pages changes to ->io_pages
 - move initializing ->ra_pages and ->io_pages for block devices to
   blk_register_queue

Changes since v2:
 - fix a rw_page return value check
 - fix up various changelogs

Changes since v1:
 - rebased to the for-5.9/block-merge branch
 - explicitly set the readahead to 0 for ubifs, vboxsf and mtd
 - split the zram block_device operations
 - let rw_page users fall back to bios in swap_readpage


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