Do not allow PBKDF benchmark over maximum allowed threads.

Unfortunatelly the benchmark function cannot return
corrected parallel cost, so it must fail.

Note that some backends (like OpenSSL) also limits maximal thread count,
so currently it was clapped at 4 for luksFormat and 8 for benchmark.

This patch set it all to PBKDF internal parallel limit.

Author: mbroz

lib/crypto_backend/pbkdf_check.c

@@ -408,6 +408,9 @@ int crypt_pbkdf_perf(const char *kdf, const char *hash,
 	if (r < 0)
 		return r;
 
+	if (parallel_threads > pbkdf_limits.max_parallel)
+		return -EINVAL;
+
 	min_memory = pbkdf_limits.min_bench_memory;
 	if (min_memory > max_memory_kb)
 		min_memory = max_memory_kb;