Fix PBKDF2 benchmark to not double iteration count for corner case.

If measurement function returns exactly 500 ms, the iteration
calculation loop doubles iteration count but instead of repeating
measurement it uses this value directly.

Thanks to Ondrej Mosnacek for bug report.

lib/crypto_backend/pbkdf_check.c

@@ -71,7 +71,7 @@ int crypt_pbkdf_check(const char *kdf, const char *hash,
 		return -ENOMEM;
 
 	iterations = 1 << 15;
-	while (ms < 500) {
+	while (1) {
 		if (getrusage(RUSAGE_SELF, &rstart) < 0) {
 			r = -EINVAL;
 			goto out;