access skb->data safely

we should use skb_header_pointer() and skb_store_bits() to access skb->data to
handle small or non-linear skbs.
Signed-off-by: Changli Gao <xiaosuo@gmail.com>
----
 net/sched/act_pedit.c |   24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)
Signed-off-by: David S. Miller <davem@davemloft.net>