dm crypt: replace open-coded kmemdup_nul

kzalloc() followed by strncpy() on an expected NUL-terminated string is
just kmemdup_nul(). Let's simplify this code (while also dropping a
deprecated strncpy() call [1]).

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-crypt.c

@@ -2858,10 +2858,9 @@ static int crypt_ctr_auth_cipher(struct crypt_config *cc, char *cipher_api)
 	if (!start || !end || ++start > end)
 		return -EINVAL;
 
-	mac_alg = kzalloc(end - start + 1, GFP_KERNEL);
+	mac_alg = kmemdup_nul(start, end - start, GFP_KERNEL);
 	if (!mac_alg)
 		return -ENOMEM;
-	strncpy(mac_alg, start, end - start);
 
 	mac = crypto_alloc_ahash(mac_alg, 0, CRYPTO_ALG_ALLOCATES_MEMORY);
 	kfree(mac_alg);