Silence some complaints from valgrind

Without this, valgrind is effectively useless in many cases, as
it floods with messages about zio writes trying to read uninited
buffers, and about the old LZ4 compressor doing the same.

It'd be nice to fix the logic that involves doing that if possible,
but for now, this at least reduces it to a small handful of messages
beyond whatever new code you're introducing.

Signed-off-by: Rich Ercolani <rincebrain@gmail.com>

module/zfs/dsl_crypt.c

@@ -1193,10 +1193,10 @@ dsl_crypto_key_sync(dsl_crypto_key_t *dck, dmu_tx_t *tx)
 {
 	zio_crypt_key_t *key = &dck->dck_key;
 	dsl_wrapping_key_t *wkey = dck->dck_wkey;
-	uint8_t keydata[MASTER_KEY_MAX_LEN];
-	uint8_t hmac_keydata[SHA512_HMAC_KEYLEN];
-	uint8_t iv[WRAPPING_IV_LEN];
-	uint8_t mac[WRAPPING_MAC_LEN];
+	uint8_t keydata[MASTER_KEY_MAX_LEN] = {0};
+	uint8_t hmac_keydata[SHA512_HMAC_KEYLEN] = {0};
+	uint8_t iv[WRAPPING_IV_LEN] = {0};
+	uint8_t mac[WRAPPING_MAC_LEN] = {0};
 
 	ASSERT(dmu_tx_is_syncing(tx));
 	ASSERT3U(key->zk_crypt, <, ZIO_CRYPT_FUNCTIONS);

module/zfs/zio.c

@@ -1706,6 +1706,15 @@ zio_write_compress(zio_t *zio)
 	if (compress != ZIO_COMPRESS_OFF &&
 	    !(zio->io_flags & ZIO_FLAG_RAW_COMPRESS)) {
 		void *cbuf = zio_buf_alloc(lsize);
+		/*
+		 * Both the crypto code and LZ4 code have bad habits of reading
+		 * uninited bytes sometimes.
+		 *
+		 * So to stop valgrind screaming to the point of uselessness,
+		 * zero them all.
+		 */
+		memset(cbuf, 0, lsize);
+
 		psize = zio_compress_data(compress, zio->io_abd, cbuf, lsize,
 		    zp->zp_complevel);
 		if (psize == 0 || psize >= lsize) {