loop: Fix discard_alignment default setting

discard_alignment is not relevant to the loop driver since it is
supposed to be set as a workaround for the old sector 63 alignments.
So set it to zero rather than block size.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reported-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>