fs/proc/task_mmu.c: check the return value of mpol_to_str()

mpol_to_str() may fail, and not fill the buffer (e.g. -EINVAL), so need
check about it, or buffer may not be zero based, and next seq_printf()
will cause issue.

The failure return need after mpol_cond_put() to match get_vma_policy().
Signed-off-by: Chen Gang <gang.chen@asianux.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -1402,8 +1402,10 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
 	walk.mm = mm;
 
 	pol = get_vma_policy(task, vma, vma->vm_start);
-	mpol_to_str(buffer, sizeof(buffer), pol);
+	n = mpol_to_str(buffer, sizeof(buffer), pol);
 	mpol_cond_put(pol);
+	if (n < 0)
+		return n;
 
 	seq_printf(m, "%08lx %s", vma->vm_start, buffer);