[SKBUFF] introduce skb_valid_link_header(skb, hdrlen)

Hi David,

	Please consider pulling from:

bk://kernel.bkbits.net/acme/sk_buff-2.6

	Nowadays only the netfilter folks use such thing, in an
open coded way, but this seems useful for other cases, so I've
moved it to skbuff.h, abstracting another direct reference to
the skb "wannabe-private" headers.

	Now there are eleven outstanding changesets in this tree.

Best Regards,

- Arnaldo