zram: always compile read_from_bdev_sync

zram: always compile read_from_bdev_sync

Patch series "zram I/O path cleanups and fixups", v3.

This series cleans up the zram I/O path, and fixes the handling of
synchronous I/O to the underlying device in the writeback_store function
or for > 4K PAGE_SIZE systems.

The fixes are at the end, as I could not fully reason about them being
safe before untangling the callchain.

This patch (of 17):

read_from_bdev_sync is currently only compiled for non-4k PAGE_SIZE, which
means it won't be built with the most common configurations.

Replace the ifdef with a check for the PAGE_SIZE in an if instead.  The
check uses an extra symbol and IS_ENABLED to allow the compiler to
eliminate the dead code, leading to the same generated code size:

   text	   data	    bss	    dec	    hex	filename
  16709	   1428	     12	  18149	   46e5	drivers/block/zram/zram_drv.o.old
  16709	   1428	     12	  18149	   46e5	drivers/block/zram/zram_drv.o.new

Link: https://lkml.kernel.org/r/20230411171459.567614-1-hch@lst.de
Link: https://lkml.kernel.org/r/20230411171459.567614-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>