latencytop: fix kernel panic while reading latency proc file

Reading /proc/<pid>/latency or /proc/<pid>/task/<tid>/latency could cause
NULL pointer dereference.

In lstats_open(), get_proc_task() can return NULL, in which case the kernel
will oops at lstats_show_proc() because m->private is NULL.

When get_proc_task() returns NULL, the kernel should return -ENOENT.

This can be reproduced by the following script.
while :
do
        date
        bash -c 'ls > ls.$$' &
        pid=$!
        cat /proc/$pid/latency &
        cat /proc/$pid/latency &
        cat /proc/$pid/latency &
        cat /proc/$pid/latency
done
Signed-off-by: NHiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

fs/proc/base.c

@@ -350,6 +350,8 @@ static int lstats_open(struct inode *inode, struct file *file)
 	struct seq_file *m;
 	struct task_struct *task = get_proc_task(inode);
 
+	if (!task)
+		return -ENOENT;
 	ret = single_open(file, lstats_show_proc, NULL);
 	if (!ret) {
 		m = file->private_data;