mkiss: fix scribble on freed memory

commit d79f16c0 fixed a user triggerable
scribble on free memory but added a new one which allows the user to
scribble even more and user controlled data into freed space.

As with 6pack we need to halt the queue before we free the buffers, because
the transmit logic is not protected by the semaphore.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>