openvswitch: Refactor ovs_flow_tbl_insert().

Rework so that ovs_flow_tbl_insert() calls flow_{key,mask}_insert().
This tidies up a future patch.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>