openvswitch: Fix ovs_flow_stats_get/clear RCU dereference.

For ovs_flow_stats_get() using ovsl_dereference() was wrong, since
flow dumps call this with RCU read lock.

ovs_flow_stats_clear() is always called with ovs_mutex, so can use
ovsl_dereference().

Also, make the ovs_flow_stats_get() 'flow' argument const to make
later patches cleaner.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>