zram: rework writeback target selection strategy

Writeback suffers from the same problem as recompression did before -
target slot selection for writeback is just a simple iteration over
zram->table entries (stored pages) which selects suboptimal targets for
writeback.  This is especially problematic for writeback, because we
uncompress objects before writeback so each of them takes 4K out of
limited writeback storage.  For example, when we take a 48 bytes slot and
store it as a 4K object to writeback device we only save 48 bytes of
memory (release from zsmalloc pool).  We naturally want to pick the
largest objects for writeback, because then each writeback will release
the largest amount of memory.

This patch applies the same solution and strategy as for recompression
target selection: pp control (post-process) with 16 buckets of candidate
pp slots.  Slots are assigned to pp buckets based on sizes - the larger
the slot the higher the group index.  This gives us sorted by size lists
of candidate slots (in linear time), so that among post-processing
candidate slots we always select the largest ones first and maximize the
memory saving.

TEST
====

A very simple demonstration: zram is configured with a writeback device. 
A limited writeback (wb_limit 2500 pages) is performed then, with a log of
sizes of slots that were written back.  You can see that patched zram
selects slots for recompression in significantly different manner, which
leads to higher memory savings (see column #2 of mm_stat output).

BASE
----

*** initial state of zram device
/sys/block/zram0/mm_stat
1750327296 619765836 631902208        0 631902208        1        0    34278    34278

*** writeback idle wb_limit 2500
/sys/block/zram0/mm_stat
1750327296 617622333 631578624        0 631902208        1        0    34278    34278

Sizes of selected objects for writeback:
... 193 349 46 46 46 46 852 1002 543 162 107 49 34 34 34 ...

PATCHED
-------

*** initial state of zram device
/sys/block/zram0/mm_stat
1750319104 619760957 631992320        0 631992320        1        0    34278    34278

*** writeback idle wb_limit 2500
/sys/block/zram0/mm_stat
1750319104 612672056 626135040        0 631992320        1        0    34278    34278

Sizes of selected objects for writeback:
... 3667 3580 3581 3580 3581 3581 3581 3231 3211 3203 3231 3246 ...

Note, pp-slots are not strictly sorted, there is a PP_BUCKET_SIZE_RANGE
variation of sizes within particular bucket.

Link: https://lkml.kernel.org/r/20240917021020.883356-5-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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