dm zoned: return NULL if dmz_get_zone_for_reclaim() fails to find a zone

dm zoned: return NULL if dmz_get_zone_for_reclaim() fails to find a zone

[ Upstream commit 489dc0f06a5837f87482c0ce61d830d24e17082e ]

The only case where dmz_get_zone_for_reclaim() cannot return a zone is
if the respective lists are empty. So we should just return a simple
NULL value here as we really don't have an error code which would make
sense.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>