dm raid: fix discard limits for raid1

Block core warned that discard_granularity was 0 for dm-raid with
personality of raid1.  Reason is that raid_io_hints() was incorrectly
special-casing raid1 rather than raid0.

Fix raid_io_hints() by removing discard limits settings for
raid1. Check for raid0 instead.

Fixes: 61697a6a ("dm: eliminate 'split_discard_bios' flag from DM target interface")
Cc: stable@vger.kernel.org
Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Reported-by: Stephan Bärwolf <stephan@matrixstorm.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>