mm/slab.c: update outdated kmem_list3 in a comment

mm/slab.c: update outdated kmem_list3 in a comment

kmem_list3 has been renamed to kmem_cache_node long long ago so update it.

References:
9beb62f35e82 ("slab: Common name for the per node structures")
d9bd0739c02b ("slab: Rename list3/l3 to node")

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Link: http://lkml.kernel.org/r/20200722033355.26908-1-yangx.jy@cn.fujitsu.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>