ceph: fix invalid point dereference for error case in mdsc destroy

1. set fsc->mdsc after successfully allocate all necessary memory
in mdsc init.
2. if fsc->mdsc is NULL, just skip destroy operation in mdsc destroy.
Signed-off-by: NChengguang Xu <cgxu519@gmx.com>
Reviewed-by: N"Yan, Zheng" <zyan@redhat.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/ceph/mds_client.c

@@ -3575,7 +3575,6 @@ int ceph_mdsc_init(struct ceph_fs_client *fsc)
 	if (!mdsc)
 		return -ENOMEM;
 	mdsc->fsc = fsc;
-	fsc->mdsc = mdsc;
 	mutex_init(&mdsc->mutex);
 	mdsc->mdsmap = kzalloc(sizeof(*mdsc->mdsmap), GFP_NOFS);
 	if (!mdsc->mdsmap) {
@@ -3583,6 +3582,7 @@ int ceph_mdsc_init(struct ceph_fs_client *fsc)
 		return -ENOMEM;
 	}
 
+	fsc->mdsc = mdsc;
 	init_completion(&mdsc->safe_umount_waiters);
 	init_waitqueue_head(&mdsc->session_close_wq);
 	INIT_LIST_HEAD(&mdsc->waiting_for_map);
@@ -3861,6 +3861,9 @@ void ceph_mdsc_destroy(struct ceph_fs_client *fsc)
 	struct ceph_mds_client *mdsc = fsc->mdsc;
 	dout("mdsc_destroy %p\n", mdsc);
 
+	if (!mdsc)
+		return;
+
 	/* flush out any connection work with references to us */
 	ceph_msgr_flush();