dp read operations depends on ovs_dp_cmd_fill_info(). This API
needs to looup vport to find dp name, but vport lookup can
fail. Therefore to keep vport reference alive we need to
take ovs lock.

Introduced by commit 6093ae9a ("openvswitch: Minimize
dp and vport critical sections").
Signed-off-by: NPravin B Shelar <pshelar@nicira.com>
Acked-by: NAndy Zhou <azhou@nicira.com>