volume: Don't unmount before deleting volume in copy

If we unmount first and then delete, the delete fails because the volume
was unmounted. Delete ends up doing the same thing as the unmount anyways.

weed/storage/disk_location.go

@@ -174,6 +174,9 @@ func (l *DiskLocation) DeleteCollectionFromDiskLocation(collection string) (e er
 }
 
 func (l *DiskLocation) deleteVolumeById(vid needle.VolumeId) (found bool, e error) {
+	l.volumesLock.Lock()
+	defer l.volumesLock.Unlock()
+
 	v, ok := l.volumes[vid]
 	if !ok {
 		return