mm: zswap: function ordering: writeback

Shrinking needs writeback. Naturally, move the writeback code above
the shrinking code. Delete the forward decl.

Link: https://lkml.kernel.org/r/20240130014208.565554-20-hannes@cmpxchg.org


Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>