exec: move de_thread()->setmax_mm_hiwater_rss() into exec_mmap()

Minor cleanup. de_thread()->setmax_mm_hiwater_rss() looks a bit
strange, move it into exec_mmap() which plays with old_mm.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -850,6 +850,7 @@ static int exec_mmap(struct mm_struct *mm)
 	if (old_mm) {
 		up_read(&old_mm->mmap_sem);
 		BUG_ON(active_mm != old_mm);
+		setmax_mm_hiwater_rss(&tsk->signal->maxrss, old_mm);
 		mm_update_next_owner(old_mm);
 		mmput(old_mm);
 		return 0;
@@ -980,9 +981,6 @@ static int de_thread(struct task_struct *tsk)
 	/* we have changed execution domain */
 	tsk->exit_signal = SIGCHLD;
 
-	if (current->mm)
-		setmax_mm_hiwater_rss(&sig->maxrss, current->mm);
-
 	exit_itimers(sig);
 	flush_itimer_signals();