net: flow_offload.h: Fix a comment at flow_action_entry.mangle

This field references FLOW_ACTION_PACKET_EDIT. Such action does not exist
though. Instead the field is used for FLOW_ACTION_MANGLE and _ADD.

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>