dm integrity: fix logic bug in integrity tag testing

If all the bytes are equal to DISCARD_FILLER, we want to accept the
buffer. If any of the bytes are different, we must do thorough
tag-by-tag checking.

The condition was inverted.

Fixes: 84597a44

 ("dm integrity: add optional discard support")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>