rbd-nbd: fix ebusy when do map

rbd-nbd: fix 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, this patch fixes it
by connecting Ceph first prior to calling NBD_SET_SOCK ioctl

Fixes: http://tracker.ceph.com/issues/23528
Signed-off-by: Li Wang <laurence.liwang@gmail.com>