zsmalloc: consolidate zs_pool's migrate_lock and size_class's locks

Currently, zsmalloc has a hierarchy of locks, which includes a pool-level
migrate_lock, and a lock for each size class.  We have to obtain both
locks in the hotpath in most cases anyway, except for zs_malloc.  This
exception will no longer exist when we introduce a LRU into the zs_pool
for the new writeback functionality - we will need to obtain a pool-level
lock to synchronize LRU handling even in zs_malloc.

In preparation for zsmalloc writeback, consolidate these locks into a
single pool-level lock, which drastically reduces the complexity of
synchronization in zsmalloc.

We have also benchmarked the lock consolidation to see the performance
effect of this change on zram.

First, we ran a synthetic FS workload on a server machine with 36 cores
(same machine for all runs), using

fs_mark  -d  ../zram1mnt  -s  100000  -n  2500  -t  32  -k

before and after for btrfs and ext4 on zram (FS usage is 80%).

Here is the result (unit is file/second):

With lock consolidation (btrfs):
Average: 13520.2, Median: 13531.0, Stddev: 137.5961482019028

Without lock consolidation (btrfs):
Average: 13487.2, Median: 13575.0, Stddev: 309.08283679298665

With lock consolidation (ext4):
Average: 16824.4, Median: 16839.0, Stddev: 89.97388510006668

Without lock consolidation (ext4)
Average: 16958.0, Median: 16986.0, Stddev: 194.7370021336469

As you can see, we observe a 0.3% regression for btrfs, and a 0.9%
regression for ext4. This is a small, barely measurable difference in my
opinion.

For a more realistic scenario, we also tries building the kernel on zram.
Here is the time it takes (in seconds):

With lock consolidation (btrfs):
real
Average: 319.6, Median: 320.0, Stddev: 0.8944271909999159
user
Average: 6894.2, Median: 6895.0, Stddev: 25.528415540334656
sys
Average: 521.4, Median: 522.0, Stddev: 1.51657508881031

Without lock consolidation (btrfs):
real
Average: 319.8, Median: 320.0, Stddev: 0.8366600265340756
user
Average: 6896.6, Median: 6899.0, Stddev: 16.04057355583023
sys
Average: 520.6, Median: 521.0, Stddev: 1.140175425099138

With lock consolidation (ext4):
real
Average: 320.0, Median: 319.0, Stddev: 1.4142135623730951
user
Average: 6896.8, Median: 6878.0, Stddev: 28.621670111997307
sys
Average: 521.2, Median: 521.0, Stddev: 1.7888543819998317

Without lock consolidation (ext4)
real
Average: 319.6, Median: 319.0, Stddev: 0.8944271909999159
user
Average: 6886.2, Median: 6887.0, Stddev: 16.93221781102523
sys
Average: 520.4, Median: 520.0, Stddev: 1.140175425099138

The difference is entirely within the noise of a typical run on zram. 
This hardly justifies the complexity of maintaining both the pool lock and
the class lock.  In fact, for writeback, we would need to introduce yet
another lock to prevent data races on the pool's LRU, further complicating
the lock handling logic.  IMHO, it is just better to collapse all of these
into a single pool-level lock.

Link: https://lkml.kernel.org/r/20221128191616.1261026-4-nphamcs@gmail.com
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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