Better document choke skb->cb[] use, like we did in netem and sfb

This adds a compile time check to make sure we dont exhaust skb->cb[]
space.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
CC: Stephen Hemminger <shemminger@vyatta.com>
CC: Patrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>