libceph: delay debugfs initialization until we learn global_id

The debugfs directory includes the cluster fsid and our unique global_id.
We need to delay the initialization of the debug entry until we have
learned both the fsid and our global_id from the monitor or else the
second client can't create its debugfs entry and will fail (and multiple
client instances aren't properly reflected in debugfs).

Reported by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>

fs/ceph/debugfs.c

@@ -201,6 +201,7 @@ int ceph_fs_debugfs_init(struct ceph_fs_client *fsc)
 	int err = -ENOMEM;
 
 	dout("ceph_fs_debugfs_init\n");
+	BUG_ON(!fsc->client->debugfs_dir);
 	fsc->debugfs_congestion_kb =
 		debugfs_create_file("writeback_congestion_kb",
 				    0600,