crypto: hmac - add hmac IPAD/OPAD constant

Many HMAC users directly use directly 0x36/0x5c values.
It's better with crypto to use a name instead of directly some crypto
constant.

This patch simply add HMAC_IPAD_VALUE/HMAC_OPAD_VALUE defines in a new
include file "crypto/hmac.h" and use them in crypto/hmac.c

Signed-off-by: Corentin Labbe <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

crypto/hmac.c

···
  *
  */
 
+#include <crypto/hmac.h>
 #include <crypto/internal/hash.h>
 #include <crypto/scatterwalk.h>
 #include <linux/err.h>
···
 	memcpy(opad, ipad, bs);
 
 	for (i = 0; i < bs; i++) {
-		ipad[i] ^= 0x36;
-		opad[i] ^= 0x5c;
+		ipad[i] ^= HMAC_IPAD_VALUE;
+		opad[i] ^= HMAC_OPAD_VALUE;
 	}
 
 	return crypto_shash_init(shash) ?:

include/crypto/hmac.h

···
+#ifndef _CRYPTO_HMAC_H
+#define _CRYPTO_HMAC_H
+
+#define HMAC_IPAD_VALUE 0x36
+#define HMAC_OPAD_VALUE 0x5c
+
+#endif /* _CRYPTO_HMAC_H */