block: remove ->rw_page

The ->rw_page method is a special purpose bypass of the usual bio handling
path that is limited to single-page reads and writes and synchronous which
causes a lot of extra code in the drivers, callers and the block layer.

The only remaining user is the MM swap code.  Switch that swap code to
simply submit a single-vec on-stack bio an synchronously wait on it based
on a newly added QUEUE_FLAG_SYNCHRONOUS flag set by the drivers that
currently implement ->rw_page instead.  While this touches one extra cache
line and executes extra code, it simplifies the block layer and drivers
and ensures that all feastures are properly supported by all drivers, e.g.
right now ->rw_page bypassed cgroup writeback entirely.

[[email protected]: fix comment typo, per Dan]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Dan Williams <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: Ira Weiny <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Keith Busch <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Vishal Verma <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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