dm integrity: test for corrupted disk format during table load

If the dm-integrity superblock was corrupted in such a way that the
journal_sections field was zero, the integrity target would deadlock
because it would wait forever for free space in the journal.

Detect this situation and refuse to activate the device.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 7eada909 ("dm: add integrity target")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>