tun: Get skfilter layout

The only thing we may have from tun device is the fprog, whic contains
the number of filter elements and a pointer to (user-space) memory
where the elements are. The program itself may not be available if the
device is persistent and detached.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>