dm-delay: fix max_delay calculations

delay_ctr() pointlessly compared max_delay in cases where multiple delay
classes were initialized identically. Also, when write delays were
configured different than read delays, delay_ctr() never compared their
value against max_delay. Fix these issues.

Fixes: 70bbeb29

 ("dm delay: for short delays, use kthread instead of timers and wq")
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>