nfsd: Drop the client reference in client_states_open()

In error path, call drop_client() to drop the reference
obtained by get_nfsdfs_clp().

Fixes: 78599c42ae3c ("nfsd4: add file to display list of client's opens")
Cc: [e-mail]
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Haoxiang Li <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/nfs4state.c

···
 		return -ENXIO;
 
 	ret = seq_open(file, &states_seq_ops);
-	if (ret)
+	if (ret) {
+		drop_client(clp);
 		return ret;
+	}
 	s = file->private_data;
 	s->private = clp;
 	return 0;