rbd-nbd: EBUSY when do map
When doing rbd-nbd map, if the Ceph service is not available,
the codes will wait on rados.connect(), unless killing the process.
In that case, the close_nbd logic is skipped with NBD_CLEAR_SOCK ioctl
not called. On the CentOS 7 kernel, it leaves nbd->file not cleared, which
causes the subsequent map requests return EBUSY