net: bridge: vlan options: move the tunnel command to the nested attribute

Now that we have a nested tunnel info attribute we can add a separate
one for the tunnel command and require it explicitly from user-space. It
must be one of RTM_SETLINK/DELLINK. Only RTM_SETLINK requires a valid
tunnel id, DELLINK just removes it if it was set before. This allows us
to have all tunnel attributes and control in one place, thus removing
the need for an outside vlan info flag.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>