Remove prefetch() from <linux/skbuff.h> and "netlabel_addrlist.h"

Commit e66eed65 ("list: remove prefetching from regular list
iterators") removed the include of prefetch.h from list.h.  The skbuff
list traversal still had them.

Quoth David Miller:
  "Please just remove the prefetches.

  Those are modelled after list.h as I intend to eventually convert
  SKB list handling to "struct list_head" but we're not there yet.

  Therefore if we kill prefetches from list.h we should kill it from
  these things in skbuff.h too."

Requested-by: David Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>