mvcc/backend: fix defrag duration scale
mvcc/backend: fix defrag duration scale

Signed-off-by: Gyuho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

cockroachdb/etcd,glevand/coreos--etcd,mqliang/etcd,xiang90/etcd,dannysauer/etcd,rnaveiras/etcd,etcd-io/etcd,rnaveiras/etcd,coreos/etcd,bdarnell/etcd,sinsharat/etcd,joshix/etcd,etcd-io/etcd,joshix/etcd,sebrandon1/etcd,li-ang/etcd,mqliang/etcd,vimalk78/etcd,ajityagaty/etcd,etcd-io/etcd,bdarnell/etcd,gyuho/etcd,sinsharat/etcd,mkumatag/etcd,mqliang/etcd,ajityagaty/etcd,glevand/coreos--etcd,vimalk78/etcd,dannysauer/etcd,mkumatag/etcd,li-ang/etcd,coreos/etcd,xiang90/etcd,gyuho/etcd,sebrandon1/etcd,ajityagaty/etcd,mkumatag/etcd,cockroachdb/etcd,rnaveiras/etcd