selftests/cgroup: add test for zswap incompressible pages

selftests/cgroup: add test for zswap incompressible pages

Add test_zswap_incompressible() to verify that the zswap_incomp memcg stat
correctly tracks incompressible pages.

The test allocates memory filled with random data from /dev/urandom, which
cannot be effectively compressed by zswap.  When this data is swapped out
to zswap, it should be stored as-is and tracked by the zswap_incomp
counter.

The test verifies that:
1. Pages are swapped out to zswap (zswpout increases)
2. Incompressible pages are tracked (zswap_incomp increases)

test:
dd if=/dev/zero of=/swapfile bs=1M count=2048
chmod 600 /swapfile
mkswap /swapfile
swapon /swapfile
echo Y > /sys/module/zswap/parameters/enabled

./test_zswap
 TAP version 13
 1..8
 ok 1 test_zswap_usage
 ok 2 test_swapin_nozswap
 ok 3 test_zswapin
 ok 4 test_zswap_writeback_enabled
 ok 5 test_zswap_writeback_disabled
 ok 6 test_no_kmem_bypass
 ok 7 test_no_invasive_cgroup_shrink
 ok 8 test_zswap_incompressible
 Totals: pass:8 fail:0 xfail:0 xpass:0 skip:0 error:0

Link: https://lkml.kernel.org/r/20260213071827.5688-3-jiayuan.chen@linux.dev
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>