crypto: xts - Replace memcpy() invocation with simple assignment

Colin reports that the memcpy() call in xts_cts_final() trigggers a
"Overlapping buffer in memory copy" warning in Coverity, which is a
false postive, given that tail is guaranteed to be smaller than or
equal to the distance between source and destination.

However, given that any additional bytes that we copy will be ignored
anyway, we can simply copy XTS_BLOCK_SIZE unconditionally, which means
we can use struct assignment of the array members instead, which is
likely to be more efficient as well.

Addresses-Coverity: ("Overlapping buffer in memory copy")
Fixes: 8083b1bf ("crypto: xts - add support for ciphertext stealing")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/xts.c

@@ -171,7 +171,7 @@ static int xts_cts_final(struct skcipher_request *req,
 				      offset - XTS_BLOCK_SIZE);
 
 	scatterwalk_map_and_copy(b, rctx->tail, 0, XTS_BLOCK_SIZE, 0);
-	memcpy(b + 1, b, tail);
+	b[1] = b[0];
 	scatterwalk_map_and_copy(b, req->src, offset, tail, 0);
 
 	le128_xor(b, &rctx->t, b);