dm verity: fix no salt use case

DM-Verity has an (undocumented) mode where no salt is used.  This was
never handled directly by the DM-Verity code, instead working due to the
fact that calling crypto_shash_update() with a zero length data is an
implicit noop.

This is no longer the case now that we have switched to
crypto_ahash_update().  Fix the issue by introducing explicit handling
of the no salt use case to DM-Verity.
Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Reported-by: Marian Csontos <mcsontos@redhat.com>
Fixes: d1ac3ff0 ("dm verity: switch to using asynchronous hash crypto API")
Tested-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-verity-target.c

@@ -166,7 +166,7 @@ static int verity_hash_init(struct dm_verity *v, struct ahash_request *req,
 		return r;
 	}
 
-	if (likely(v->version >= 1))
+	if (likely(v->salt_size && (v->version >= 1)))
 		r = verity_hash_update(v, req, v->salt, v->salt_size, res);
 
 	return r;
@@ -177,7 +177,7 @@ static int verity_hash_final(struct dm_verity *v, struct ahash_request *req,
 {
 	int r;
 
-	if (unlikely(!v->version)) {
+	if (unlikely(v->salt_size && (!v->version))) {
 		r = verity_hash_update(v, req, v->salt, v->salt_size, res);
 
 		if (r < 0) {