lib/show_mem.c: use for_each_populated_zone() simplify code

lib/show_mem.c: use for_each_populated_zone() simplify code

__show_mem() needs to iterate over all zones that have memory, we can
simplify the code by using for_each_populated_zone().

Link: https://lkml.kernel.org/r/20230417035226.4013584-1-yajun.deng@linux.dev
Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>