tun: add tun_flags, owner, group attributes in sysfs

This patch adds three attribute files in /sys/class/net/$dev/ for tun
devices; allowing userspace to obtain the information which TUNGETIFF
offers, and more, but without having to attach to the device in question
(which may not be possible if it's in use).

It also fixes a bug which has been present in the TUNGETIFF ioctl since
its inception, where it would never set IFF_TUN or IFF_TAP according to
the device type. (Look carefully at the code which I remove from
tun_get_iff() and how the new tun_flags() helper is subtly different).

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>