dm integrity: add optional discard support

Add an argument "allow_discards" that enables discard processing on
dm-integrity device. Discards are only allowed to devices using
internal hash.

When a block is discarded the integrity tag is filled with
DISCARD_FILLER (0xf6) bytes.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>