bridge: new function to pack vlans into ranges during gets

This patch adds new function to pack vlans into ranges
whereever applicable using the flags BRIDGE_VLAN_INFO_RANGE_BEGIN
and BRIDGE VLAN_INFO_RANGE_END

Old vlan packing code is moved to a new function and continues to be
called when filter_mask is RTEXT_FILTER_BRVLAN.

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