selftests: fix the zswap invasive shrink test

The zswap no invasive shrink selftest breaks because we rename the zswap
writeback counter (see [1]).  Fix the test.

[1]: https://patchwork.kernel.org/project/linux-kselftest/patch/20231205193307.2432803-1-nphamcs@gmail.com/

Link: https://lkml.kernel.org/r/20240205225608.3083251-3-nphamcs@gmail.com
Fixes: a697dc2b ("selftests: cgroup: update per-memcg zswap writeback selftest")
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/cgroup/test_zswap.c

@@ -52,7 +52,7 @@ static int get_zswap_stored_pages(size_t *value)
 
 static int get_cg_wb_count(const char *cg)
 {
-	return cg_read_key_long(cg, "memory.stat", "zswp_wb");
+	return cg_read_key_long(cg, "memory.stat", "zswpwb");
 }
 
 static long get_zswpout(const char *cgroup)