exec: Run sync_mm_rss before taking exec_update_mutex

Like exec_mm_release sync_mm_rss is about flushing out the state of
the old_mm, which does not need to happen under exec_update_mutex.

Make this explicit by moving sync_mm_rss outside of exec_update_mutex.
Reviewed-by: NKees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/875zd66za3.fsf_-_@x220.int.ebiederm.orgSigned-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

fs/exec.c

@@ -1051,13 +1051,14 @@ static int exec_mmap(struct mm_struct *mm)
 	tsk = current;
 	old_mm = current->mm;
 	exec_mm_release(tsk, old_mm);
+	if (old_mm)
+		sync_mm_rss(old_mm);
 
 	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
 	if (ret)
 		return ret;
 
 	if (old_mm) {
-		sync_mm_rss(old_mm);
 		/*
 		 * Make sure that if there is a core dump in progress
 		 * for the old mm, we get out and die instead of going