[NET]: Zerocopy sequential reading of skb data

Implements sequential reading for both linear and non-linear
skb data at zerocopy cost. The data is returned in chunks of
arbitary length, therefore random access is not possible.

Usage:
	from	 := 0
	to	 := 128
	state	 := undef
	data	 := undef
	len	 := undef
	consumed := 0

	skb_prepare_seq_read(skb, from, to, &state)
	while (len = skb_seq_read(consumed, &data, &state)) != 0 do
		/* do something with 'data' of length 'len' */
		if abort then
			/* abort read if we don't wait for
			 * skb_seq_read() to return 0 */
			skb_abort_seq_read(&state)
			return
		endif
		/* not necessary to consume all of 'len' */
		consumed += len
	done

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>