DVR: Optimize getting arp entry info
DVR: Optimize getting arp entry info

Grab duplicated code in add_arp_entry and del_arp_entry.

Change-Id: I579d66ec734627dec08df6fba750a3e237600a5c

bigswitch/neutron,eayunstack/neutron,sebrandon1/neutron,igor-toga/local-snat,huntxu/neutron,eayunstack/neutron,openstack/neutron,dims/neutron,wolverineav/neutron,noironetworks/neutron,mahak/neutron,igor-toga/local-snat,wolverineav/neutron,bigswitch/neutron,MaximNevrov/neutron,mahak/neutron,klmitch/neutron,openstack/neutron,openstack/neutron,noironetworks/neutron,dims/neutron,cloudbase/neutron,MaximNevrov/neutron,cloudbase/neutron,huntxu/neutron,sebrandon1/neutron,klmitch/neutron,mahak/neutron