crypto: null - Add default null skcipher

This patch adds a default null skcipher for users such as gcm
to perform copies on SG lists.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
diff --git a/include/crypto/null.h b/include/crypto/null.h
index b7c864c..06dc30d 100644
--- a/include/crypto/null.h
+++ b/include/crypto/null.h
@@ -8,4 +8,7 @@
 #define NULL_DIGEST_SIZE	0
 #define NULL_IV_SIZE		0
 
+struct crypto_blkcipher *crypto_get_default_null_skcipher(void);
+void crypto_put_default_null_skcipher(void);
+
 #endif