NFS: Cleanup error handling in nfs_idmap_request_key()

nfs_idmap_get_desc() can't actually return zero.  But if it did then
we would return ERR_PTR(0) which is NULL and the caller,
nfs_idmap_get_key(), doesn't expect that so it leads to a NULL pointer
dereference.

I've cleaned this up by changing the "<=" to "<" so it's more clear that
we don't return ERR_PTR(0).
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/nfs4idmap.c

@@ -274,7 +274,7 @@ static struct key *nfs_idmap_request_key(const char *name, size_t namelen,
 	ssize_t ret;
 
 	ret = nfs_idmap_get_desc(name, namelen, type, strlen(type), &desc);
-	if (ret <= 0)
+	if (ret < 0)
 		return ERR_PTR(ret);
 
 	rkey = request_key(&key_type_id_resolver, desc, "");