[NET]: Revert sk_buff walker cleanups.

This reverts eefa3906

The simplification made in that change works with the assumption that
the 'offset' parameter to these functions is always positive or zero,
which is not true.  It can be and often is negative in order to access
SKB header values in front of skb->data.

Signed-off-by: David S. Miller <davem@davemloft.net>