flow_dissector: Dissect layer 2 miss from tc skb extension

Extend the 'FLOW_DISSECTOR_KEY_META' key with a new 'l2_miss' field and
populate it from a field with the same name in the tc skb extension.
This field is set by the bridge driver for packets that incur an FDB or
MDB miss.

The next patch will extend the flower classifier to be able to match on
layer 2 misses.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>