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>