There's no convenient function to walk the data of an skbuff; provide
one.  Netfilter extensions in particular can use this to examine
packet contents without needing an entire copy.

I originally wrote an 'skb_walk(skb, fn, data)' function, but open
iterators are easier to use for complex cases.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@redhat.com>