tracing/stack-tracer: avoid races accessing file

Impact: fix race

vma->vm_file reference is only stable while holding the mmap_sem,
so move usage of it to within the critical section.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace.c

@@ -1470,13 +1470,13 @@ static inline int seq_print_user_ip(struct trace_seq *s, struct mm_struct *mm,
 			file = vma->vm_file;
 			vmstart = vma->vm_start;
 		}
-		up_read(&mm->mmap_sem);
-	}
 		if (file) {
 			ret = trace_seq_path(s, &file->f_path);
 			if (ret)
 				ret = trace_seq_printf(s, "[+0x%lx]", ip - vmstart);
 		}
+		up_read(&mm->mmap_sem);
+	}
 	if (ret && ((sym_flags & TRACE_ITER_SYM_ADDR) || !file))
 		ret = trace_seq_printf(s, " <" IP_FMT ">", ip);
 	return ret;