The __read_swap_cache_async() interface isn't more difficult to understand
than what the helper abstracts.  Save the indirection and a level of
indentation for the primary work of the writeback func.

Link: https://lkml.kernel.org/r/20230727162343.1415598-4-hannes@cmpxchg.orgSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Barry Song <song.bao.hua@hisilicon.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>