net: bridge: vlan: add del rtm message support

Adding RTM_DELVLAN support similar to RTM_NEWVLAN is simple, just need to
map DELVLAN to DELLINK and register the handler.

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