[NET] skbuff: skb_store_bits const is backwards

Getting warnings becuase skb_store_bits has skb as constant,
but the function overwrites it. Looks like const was on the
wrong side.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>