page_counter: reduce struct page_counter size

The struct page_counter has explicit padding for better cache alignment. 
The commit c6f53ed ("mm, memcg: cg2 memory{.swap,}.peak write
handlers") added a field to the struct page_counter and accidently
increased its size.  Let's move the failcnt field which is v1-only field
to the same cacheline of usage to reduce the size of struct page_counter.

Link: https://lkml.kernel.org/r/20250228075808.207484-4-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: shakeelb

include/linux/page_counter.h

@@ -9,10 +9,12 @@
 
 struct page_counter {
 	/*
-	 * Make sure 'usage' does not share cacheline with any other field. The
-	 * memcg->memory.usage is a hot member of struct mem_cgroup.
+	 * Make sure 'usage' does not share cacheline with any other field in
+	 * v2. The memcg->memory.usage is a hot member of struct mem_cgroup.
 	 */
 	atomic_long_t usage;
+	unsigned long failcnt; /* v1-only field */
+
 	CACHELINE_PADDING(_pad1_);
 
 	/* effective memory.min and memory.min usage tracking */
@@ -28,7 +30,6 @@ struct page_counter {
 	unsigned long watermark;
 	/* Latest cg2 reset watermark */
 	unsigned long local_watermark;
-	unsigned long failcnt; /* v1-only field */
 
 	/* Keep all the read most fields in a separete cacheline. */
 	CACHELINE_PADDING(_pad2_);