mm: memcontrol: remove unnecessary NULL check before free_percpu()

mm: memcontrol: remove unnecessary NULL check before free_percpu()

free_percpu() checks for NULL pointers internally.  Remove unneeded NULL
check here.

Link: https://lkml.kernel.org/r/20250417084330.937380-1-nichen@iscas.ac.cn
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
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: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>