tun: send netlink notification when the device is modified

I added dumping of link information about tun devices over netlink in
commit 1ec010e7

 ("tun: export flags, uid, gid, queue information
over netlink"), but didn't add the missing netlink notifications when
the device's exported properties change.

This patch adds notifications when owner/group or flags are modified,
when queues are attached/detached, and when a tun fd is closed.

Reported-by: Thomas Haller <thaller@redhat.com>
Fixes: 1ec010e7

 ("tun: export flags, uid, gid, queue information over netlink")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>