Fix a warning false positive from clang 10.

blf_enc() takes a number of 64-bit blocks to encrypt, but using
sizeof(uint64_t) in the calculation triggers a warning from clang
10 because the actual data type is uint32_t.  Pass BCRYPT_WORDS / 2
for the number of blocks like libc bcrypt(3) does.  OK kettenis@

sys/lib/libsa/bcrypt_pbkdf.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: bcrypt_pbkdf.c,v 1.1 2016/09/10 18:26:28 jsing Exp $ */
+/* $OpenBSD: bcrypt_pbkdf.c,v 1.2 2020/07/09 19:17:19 millert Exp $ */
 /*
  * Copyright (c) 2013 Ted Unangst <tedu@openbsd.org>
  *
@@ -76,7 +76,7 @@ bcrypt_hash(uint8_t *sha2pass, uint8_t *sha2salt, uint8_t *out)
 		cdata[i] = Blowfish_stream2word(ciphertext, sizeof(ciphertext),
 		    &j);
 	for (i = 0; i < 64; i++)
-		blf_enc(&state, cdata, sizeof(cdata) / sizeof(uint64_t));
+		blf_enc(&state, cdata, BCRYPT_WORDS / 2);
 
 	/* copy out */
 	for (i = 0; i < BCRYPT_WORDS; i++) {