tun: keep link (carrier) state up to date

Currently, only ethtool can get accurate link state of a tap device.
With this patch, IFF_RUNNING and IF_OPER_UP/DOWN are kept up to date as
well.

Signed-off-by: Nolan Leake <nolan@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>