iommu/iova: Separate atomic variables to improve performance

In struct iova_domain, there are three atomic variables, the former two
are about TLB flush counters which use atomic_add operation, anoter is
used to flush timer that use cmpxhg operation.
These variables are in the same cache line, so it will cause some
performance loss under the condition that many cores call queue_iova
function, Let's isolate the two type atomic variables to different
cache line to reduce cache line conflict.

Cc: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Jinyu Qi <jinyuqi@huawei.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>