[NET]: Document some simple rules for actions

The "enviromental" rules for authors of any new tc actions are:

1) If you stealeth or borroweth any packet thou shalt be branching

from the righteous path and thou shalt cloneth.

For example if your action queues a packet to be processed later

or intentionaly branches by redirecting a packet then you need to

clone the packet.

There are certain fields in the skb tc_verd that need to be reset so we

avoid loops etc. A few are generic enough so much so that skb_act_clone()

resets them for you. So invoke skb_act_clone() rather than skb_clone()

2) If you munge any packet thou shalt call pskb_expand_head in the case

someone else is referencing the skb. After that you "own" the skb.

You must also tell us if it is ok to munge the packet (TC_OK2MUNGE),

this way any action downstream can stomp on the packet.

3) dropping packets you dont own is a nono. You simply return

TC_ACT_SHOT to the caller and they will drop it.

The "enviromental" rules for callers of actions (qdiscs etc) are:

*) thou art responsible for freeing anything returned as being

TC_ACT_SHOT/STOLEN/QUEUED. If none of TC_ACT_SHOT/STOLEN/QUEUED is

returned then all is great and you dont need to do anything.

Post on netdev if something is unclear.