Since crypto API commit 9fa68f62 ("crypto: hash - prevent using keyed
hashes without setting key") dm-integrity cannot use keyed algorithms
without the key being set.

The dm-integrity recognizes this too late (during use of HMAC), so it
allows creation and formatting of superblock, but the device is in fact
unusable.

Fix it by detecting the key requirement in integrity table constructor.
Signed-off-by: NMilan Broz <gmazyland@gmail.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>