devpts: return NULL pts 'priv' entry for non-devpts nodes

devpts: return NULL pts 'priv' entry for non-devpts nodes

In commit 6b1a03eab323 ("devpts: more pty driver interface cleanups") I
made devpts_get_priv() just return the dentry->fs_data directly.  And
because I thought it wouldn't happen, I added a warning if you ever saw
a pts node that wasn't on devpts.

And no, that warning never triggered under any actual real use, but you
can trigger it by creating nonsensical pts nodes by hand.

So just revert the warning, and make devpts_get_priv() return NULL for
that case like it used to.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org # 4.6+
Cc: Eric W Biederman" <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>