openvswitch: Fix race.

ovs_vport_cmd_dump() did rcu_read_lock() only after getting the
datapath, which could have been deleted in between.  Resolved by
taking rcu_read_lock() before the get_dp() call.

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