mm/gup: Mark lock taken only after a successful retake

It's definitely incorrect to mark the lock as taken even if
down_read_killable() failed.

This wass overlooked when we switched from down_read() to
down_read_killable() because down_read() won't fail while
down_read_killable() could.

Fixes: 71335f37 ("mm/gup: allow to react to fatal signals")
Reported-by: syzbot+a8c70b7f3579fc0587dc@syzkaller.appspotmail.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/gup.c

@@ -1329,7 +1329,6 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 		if (fatal_signal_pending(current))
 			break;
 
-		*locked = 1;
 		ret = down_read_killable(&mm->mmap_sem);
 		if (ret) {
 			BUG_ON(ret > 0);
@@ -1338,6 +1337,7 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 			break;
 		}
 
+		*locked = 1;
 		ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
 				       pages, NULL, locked);
 		if (!*locked) {