datapath: Convert dp rcu read operation to locked operations

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.

Found by code inspection.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>