net: flow_dissector: add support for cfm packets

Add support for dissecting cfm packets. The cfm packet header
fields maintenance domain level and opcode can be dissected.

Signed-off-by: Zahari Doychev <zdoychev@maxlinear.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>