net: rps: reset network header before calling skb_get_rxhash()

skb_get_rxhash() assumes the network header pointer of the skb is set
properly 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>