memcg: make count_memcg_events re-entrant safe against irqs

Let's make count_memcg_events re-entrant safe against irqs.  The only
thing needed is to convert the usage of __this_cpu_add() to
this_cpu_add().  In addition, with re-entrant safety, there is no need to
disable irqs.  Also add warnings for in_nmi() as it is not safe against
nmi context.

Link: https://lkml.kernel.org/r/20250514184158.3471331-5-shakeel.butt@linux.dev


Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>