net: bridge: vlan: include stats in dumps if requested

This patch adds support for vlan stats to be included when dumping vlan
information. We have to dump them only when explicitly requested (thus the
flag below) because that disables the vlan range compression and will make
the dump significantly larger. In order to request the stats to be
included we add a new dump attribute called BRIDGE_VLANDB_DUMP_FLAGS which
can affect dumps with the following first flag:
  - BRIDGE_VLANDB_DUMPF_STATS
The stats are intentionally nested and put into separate attributes to make
it easier for extending later since we plan to add per-vlan mcast stats,
drop stats and possibly STP stats. This is the last missing piece from the
new vlan API which makes the dumped vlan information complete.

A dump request which should include stats looks like:
 [BRIDGE_VLANDB_DUMP_FLAGS] |= BRIDGE_VLANDB_DUMPF_STATS

A vlandb entry attribute with stats looks like:
 [BRIDGE_VLANDB_ENTRY] = {
     [BRIDGE_VLANDB_ENTRY_STATS] = {
         [BRIDGE_VLANDB_STATS_RX_BYTES]
         [BRIDGE_VLANDB_STATS_RX_PACKETS]
         ...
     }
 }

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