datapath: Improve compat rxhash functionality.

Following patch improves rxhash calculation, It is taken from
upstream Linux kernel code.
From kernel 3.8, skb_get_rxhash() can handle hardware generated
l4-rxhash. Therefore compat skb_get_rxhash() is not used on
kernel 3.8 or new.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Reviewed-by: Thomas Graf <tgraf@redhat.com>
Acked-by: Jesse Gross <jesse@nicira.com>