rtnetlink: new filter RTEXT_FILTER_BRVLAN_COMPRESSED

This filter is same as RTEXT_FILTER_BRVLAN except that it tries
to compress the consecutive vlans into ranges.

This helps on systems with large number of configured vlans.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>