tun: impossible to deassert IFF_ONE_QUEUE or IFF_NO_PI

From: "Nathaniel Filardo" <nwfilardo@gmail.com>

Taken from http://bugzilla.kernel.org/show_bug.cgi?id=9806

The TUN/TAP driver only permits one-way transitions of IFF_NO_PI or
IFF_ONE_QUEUE during the lifetime of a tap/tun interface.  Note that
tun_set_iff contains

 541         if (ifr->ifr_flags & IFF_NO_PI)
 542                 tun->flags |= TUN_NO_PI;
 543 
 544         if (ifr->ifr_flags & IFF_ONE_QUEUE)
 545                 tun->flags |= TUN_ONE_QUEUE;

This is easily fixed by adding else branches which clear these bits.

Steps to reproduce:

This is easily reproduced by setting an interface persistant using tunctl then
attempting to open it as IFF_TAP or IFF_TUN, without asserting the IFF_NO_PI
flag.  The ioctl() will succeed and the ifr.flags word is not modified, but the
interface remains in IFF_NO_PI mode (as it was set by tunctl).

Acked-by: Maxim Krasnyansky <maxk@qualcomm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>