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>