wdp-xflow: Copy client_data from old rule to new rule when displacing a rule.

wdp-xflow: Copy client_data from old rule to new rule when displacing a rule.

If client_data is not copied when a new rule displaces an old one, then
the old rule's client data will be leaked and the new rule will have a null
pointer.  This causes a segfault with, e.g.:

    ovs-ofctl add-flow br0 "dl_type=0x800,nw_dst=10.245.7.0/16,actions=output:148"
    ovs-ofctl mod-flows --strict br0 "dl_type=0x800,nw_dst=10.245.7.0/16,actions=output:149"
    ovs-ofctl del-flows --strict br0 "dl_type=0x800,nw_dst=10.245.7.0/16"

Reported by partner.