After page was truncated it lost it's mapping, this result in null
pointer dereference on bdi_stat update. In fact we have to decrement
bdi_stat even for truncated pages, so let's pass correct mapping in
function arguments. Patch against linux-2.6
##TEST_CASE
/*
Tast case for bug in nfs_clear_request_commit()
caused by null pointer dereference in case of truncated page.
It takes less than 10 minutes to reproduce the bug.
### start script
#! /bin/bash -x
# mount my-host:/my-share /mnt
mkdir /mnt/T
cd /mnt/T || exit 1
while true ;
      do for ((i=0;i<3;i++)); do
      	 /tmp/mmap file3 200000000 $i & done;
	 sleep 2 ;
	 killall -9 mmap ;
      done
done
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
int main(int argc, char *argv[])
{
	char *addr;
	unsigned int size;
	off_t off;
	int i = 0;
	int fd, result;
	if (argc != 4) {
		perror("Wrong args:\n Usage: %s [file] [size] [offset]\n");
		exit(EXIT_FAILURE);
	}
	size = atoi(argv[2]);
	size &= ~(4096-1);
	off = atol(argv[3]);
	off = ((off_t)size)*off;
	fd = open(argv[1], O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600);
	if (fd == -1) {
		perror("Error opening file for writing");
		exit(EXIT_FAILURE);
	}
	result = lseek(fd, off + (off_t)size, SEEK_SET);
	if (result == -1) {
		close(fd);
		perror("Error calling lseek() to 'stretch' the file");
		exit(EXIT_FAILURE);
	}
	write(fd, "a", 1);
	addr = mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, off);
	if (addr == MAP_FAILED) {
		close(fd);
		perror("Error mmapping the file");
		exit(EXIT_FAILURE);
	}
	/* Now write int's to the file as if it were memory (an array of ints).
	 */
	while (1) {
		memset(addr, i++, size);
	}
}