tun: replace tun_debug() by netif_info()

The tun driver uses custom macro tun_debug() which is only available if
TUN_DEBUG is set. Replace it by standard netif_ifinfo(). For that purpose,
rename tun_struct::debug to msg_enable and make it u32 and always present.
Finally, make tun_get_msglevel(), tun_set_msglevel() and TUNSETDEBUG ioctl
independent of TUN_DEBUG.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>