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: a739ff3f543af ("dm verity: add support for forward error correction")
Cc: [e-mail]
Signed-off-by: Sunwook Eom <[e-mail]>
Reviewed-by: Sami Tolvanen <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-verity-fec.c

···
 	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