mlxsw: spectrum_flower: Offload FLOW_ACTION_MANGLE

Offload action pedit ex munge when used with a flower classifier. Only
allow setting of DSCP, ECN, or the whole DSField in IPv4 and IPv6 packets.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>