Mlocked pages statistics shows bogus value.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

While reading OOM report from Jan Stancek, I noticed that
NR_MLOCK statistics shows bogus values.


Steps to reproduce:

(1) Check Mlocked: field of /proc/meminfo or mlocked: field of SysRq-m.

(2) Compile and run below program with appropriate size as argument.
    There is no need to invoke the OOM killer.

----------
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>

int main(int argc, char *argv[])
{
	unsigned long length = atoi(argv[1]);
	void *addr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
	if (addr == MAP_FAILED)
		printf("mmap() failed\n");
	else if (mlock(addr, length) == -1)
		printf("mlock() failed\n");
	else
		printf("MLocked %lu bytes\n", length);
	return 0;
}
----------

(3) Check Mlocked: field or mlocked: field again.
    You can see the value became very large due to
    NR_MLOCK counter going negative.

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