crypto: gcm - add GCM IV size constant

Many GCM users use directly GCM IV size instead of using some constant.

This patch add all IV size constant used by GCM.
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/gcm.h

+#ifndef _CRYPTO_GCM_H
+#define _CRYPTO_GCM_H
+
+#define GCM_AES_IV_SIZE 12
+#define GCM_RFC4106_IV_SIZE 8
+#define GCM_RFC4543_IV_SIZE 8
+
+#endif