mm: use the same log level for show_mem()

Use the same log level for printk's in show_mem(), so that those messages
can be shown completely when using log level 6.

Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>