dm integrity: revert to not using discard filler when recalulating

Revert the commit 7a5b96b4 ("dm integrity:
use discard support when recalculating").

There's a bug that when we write some data beyond the current recalculate
boundary, the checksum will be rewritten with the discard filler later.
And the data will no longer have integrity protection. There's no easy
fix for this case.

Also, another problematic case is if dm-integrity is used to detect
bitrot (random device errors, bit flips, etc); dm-integrity should
detect that even for unused sectors. With commit 7a5b96b4 it can
happen that such change is undetected (because discard filler is not a
valid checksum).

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>