crypto: algapi - make crypto_xor() take separate dst and src arguments

There are quite a number of occurrences in the kernel of the pattern

  if (dst != src)
          memcpy(dst, src, walk.total % AES_BLOCK_SIZE);
  crypto_xor(dst, final, walk.total % AES_BLOCK_SIZE);

or

  crypto_xor(keystream, src, nbytes);
  memcpy(dst, keystream, nbytes);

where crypto_xor() is preceded or followed by a memcpy() invocation
that is only there because crypto_xor() uses its output parameter as
one of the inputs. To avoid having to add new instances of this pattern
in the arm64 code, which will be refactored to implement non-SIMD
fallbacks, add an alternative implementation called crypto_xor_cpy(),
taking separate input and output arguments. This removes the need for
the separate memcpy().

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/ctr.c

@@ -65,8 +65,7 @@ static void crypto_ctr_crypt_final(struct blkcipher_walk *walk,
 	unsigned int nbytes = walk->nbytes;
 
 	crypto_cipher_encrypt_one(tfm, keystream, ctrblk);
-	crypto_xor(keystream, src, nbytes);
-	memcpy(dst, keystream, nbytes);
+	crypto_xor_cpy(dst, keystream, src, nbytes);
 
 	crypto_inc(ctrblk, bsize);
 }