mm: zswap: function ordering: pool alloc & free

The function ordering in zswap.c is a little chaotic, which requires
jumping in unexpected directions when following related code. This is
a series of patches that brings the file into the following order:

- pool functions
- lru functions
- rbtree functions
- zswap entry functions
- compression/backend functions
- writeback & shrinking functions
- store, load, invalidate, swapon, swapoff
- debugfs
- init

But it has to be split up such the moving still produces halfway
readable diffs.

In this patch, move pool allocation and freeing functions.

Link: https://lkml.kernel.org/r/20240130014208.565554-11-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>