nfsd: call cache_put if xdr_reserve_space returns NULL

If not enough buffer space available, but idmap_lookup has triggered
lookup_fn which calls cache_get and returns successfully. Then we
missed to call cache_put here which pairs with cache_get.

Fixes: ddd1ea56 ("nfsd4: use xdr_reserve_space in attribute encoding")
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Reviwed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4idmap.c

@@ -581,6 +581,7 @@ static __be32 idmap_id_to_name(struct xdr_stream *xdr,
 		.id = id,
 		.type = type,
 	};
+	__be32 status = nfs_ok;
 	__be32 *p;
 	int ret;
 	struct nfsd_net *nn = net_generic(SVC_NET(rqstp), nfsd_net_id);
@@ -593,12 +594,16 @@ static __be32 idmap_id_to_name(struct xdr_stream *xdr,
 		return nfserrno(ret);
 	ret = strlen(item->name);
 	WARN_ON_ONCE(ret > IDMAP_NAMESZ);
+
 	p = xdr_reserve_space(xdr, ret + 4);
-	if (!p)
-		return nfserr_resource;
-	p = xdr_encode_opaque(p, item->name, ret);
+	if (unlikely(!p)) {
+		status = nfserr_resource;
+		goto out_put;
+	}
+	xdr_encode_opaque(p, item->name, ret);
+out_put:
 	cache_put(&item->h, nn->idtoname_cache);
-	return 0;
+	return status;
 }
 
 static bool