net: rps: fix the wrong network header pointer

__skb_get_rxhash() was broken after the commit:

 commit bfb564e7
 Author: Krishna Kumar <krkumar2@in.ibm.com>
 Date:   Wed Aug 4 06:15:52 2010 +0000

 core: Factor out flow calculation from get_rps_cpu

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>