picker: Fix data race in PickAddr
picker: Fix data race in PickAddr

PickAddr sets p.peer to peer, then returns p.peer.Addr after unlocking
its mutex. It should return peer.Addr instead to avoid triggering a data
race.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

deljuven/swarmkit,deljuven/swarmkit,allencloud/swarmkit,cezarsa/swarmkit,sungwonh/swarmkit,vasil-yordanov/swarmkit,RenaudWasTaken/swarmkit,vasil-yordanov/swarmkit,Noyan759/swarmkit,jlhawn/swarmkit,fate-grand-order/swarmkit,sungwonh/swarmkit,jlhawn/swarmkit,docker/swarmkit,yongtang/swarmkit,diogomonica/swarmkit,aaronlehmann/swarmkit,fate-grand-order/swarmkit,dperny/swarmkit-1,AliyunContainerService/swarmkit,RenaudWasTaken/swarmkit,cheyang/swarmkit,AliyunContainerService/swarmkit,dperny/swarmkit-1,yp-engineering/swarmkit,elifa/swarmkit,Noyan759/swarmkit,alyyousuf7/swarmkit,aaronlehmann/swarmkit,cezarsa/swarmkit,cheyang/swarmkit,diogomonica/swarmkit,docker/swarmkit,alyyousuf7/swarmkit,elifa/swarmkit,yp-engineering/swarmkit,allencloud/swarmkit,yongtang/swarmkit