dm verity fec: fix hash block number in verity_fec_decode

The error correction data is computed as if data and hash blocks
were concatenated. But hash block number starts from v->hash_start.
So, we have to calculate hash block number based on that.

Fixes: a739ff3f ("dm verity: add support for forward error correction")
Cc: stable@vger.kernel.org
Signed-off-by: Sunwook Eom <speed.eom@samsung.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-verity-fec.c

@@ -435,7 +435,7 @@ int verity_fec_decode(struct dm_verity *v, struct dm_verity_io *io,
 	fio->level++;
 
 	if (type == DM_VERITY_BLOCK_TYPE_METADATA)
-		block += v->data_blocks;
+		block = block - v->hash_start + v->data_blocks;
 
 	/*
 	 * For RS(M, N), the continuous FEC data is divided into blocks of N