ceph: pass ino# instead of old_dentry if it's disconnected

When exporting the kceph to NFS it may pass a DCACHE_DISCONNECTED
dentry for the link operation. Then it will parse this dentry as a
snapdir, and the mds will fail the link request as -EROFS.

MDS allow clients to pass a ino# instead of a path.

Link: https://tracker.ceph.com/issues/59515
Signed-off-by: Xiubo Li <[e-mail]>
Reviewed-by: Ilya Dryomov <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

fs/ceph/dir.c

···
 	struct ceph_mds_request *req;
 	int err;
 
+	if (dentry->d_flags & DCACHE_DISCONNECTED)
+		return -EINVAL;
+
 	err = ceph_wait_on_conflict_unlink(dentry);
 	if (err)
 		return err;
···
 	if (ceph_snap(dir) != CEPH_NOSNAP)
 		return -EROFS;
 
-	dout("link in dir %p old_dentry %p dentry %p\n", dir,
-	     old_dentry, dentry);
+	dout("link in dir %p %llx.%llx old_dentry %p:'%pd' dentry %p:'%pd'\n",
+	     dir, ceph_vinop(dir), old_dentry, old_dentry, dentry, dentry);
 	req = ceph_mdsc_create_request(mdsc, CEPH_MDS_OP_LINK, USE_AUTH_MDS);
 	if (IS_ERR(req)) {
 		d_drop(dentry);
···
 	req->r_dentry = dget(dentry);
 	req->r_num_caps = 2;
 	req->r_old_dentry = dget(old_dentry);
+	/*
+	 * The old_dentry maybe a DCACHE_DISCONNECTED dentry, then we
+	 * will just pass the ino# to MDSs.
+	 */
+	if (old_dentry->d_flags & DCACHE_DISCONNECTED)
+		req->r_ino2 = ceph_vino(d_inode(old_dentry));
 	req->r_parent = dir;
 	ihold(dir);
 	set_bit(CEPH_MDS_R_PARENT_LOCKED, &req->r_req_flags);

fs/ceph/mds_client.c

···
 	u64 ino1 = 0, ino2 = 0;
 	int pathlen1 = 0, pathlen2 = 0;
 	bool freepath1 = false, freepath2 = false;
+	struct dentry *old_dentry = NULL;
 	int len;
 	u16 releases;
 	void *p, *end;
···
 	}
 
 	/* If r_old_dentry is set, then assume that its parent is locked */
-	ret = set_request_path_attr(NULL, req->r_old_dentry,
+	if (req->r_old_dentry &&
+	    !(req->r_old_dentry->d_flags & DCACHE_DISCONNECTED))
+		old_dentry = req->r_old_dentry;
+	ret = set_request_path_attr(NULL, old_dentry,
 			      req->r_old_dentry_dir,
 			      req->r_path2, req->r_ino2.ino,
 			      &path2, &pathlen2, &ino2, &freepath2, true);