mm: memcontrol: drop superfluous entry in the per-memcg stats array

MEM_CGROUP_STAT_NSTATS is just a delimiter for cgroup1 statistics, not
an actual array entry. Reuse it for the first cgroup2 stat entry, like
in the event array.

Fixes: b2807f07f4f8 ("mm: memcontrol: add "sock" to cgroup2 memory.stat")
Signed-off-by: Johannes Weiner <[e-mail]>
Cc: Vladimir Davydov <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/memcontrol.h

···
 	MEM_CGROUP_STAT_SWAP,		/* # of pages, swapped out */
 	MEM_CGROUP_STAT_NSTATS,
 	/* default hierarchy stats */
-	MEMCG_SOCK,
+	MEMCG_SOCK = MEM_CGROUP_STAT_NSTATS,
 	MEMCG_NR_STAT,
 };