blk-throttle: implement throtl_grp->has_rules[]

blk_throtl_bio() has a quick exit path for throtl_grps without limits
configured.  It looks at the bps and iops limits and if both are not
configured, the bio is issued immediately.  While this is correct in
the current flat hierarchy as each throtl_grp behaves completely
independently, it would become wrong in proper hierarchy mode.  A
group without any limits could still be limited by one of its
ancestors and bio's queued for such group should not bypass
blk-throtl.

As having a quick bypass mechanism is beneficial, this patch
reimplements the mechanism such that it's correct even with proper
hierarchy.  throtl_grp->has_rules[] is added.  These booleans are
updated for the whole subtree whenever a config is updated so that
has_rules[] of the whole subtree stays synchronized.  They're also
updated when a new throtl_grp comes online so that it can't escape the
limits of its ancestors.

As no throtl_grp has another throtl_grp as parent now, this patch
doesn't yet make any behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>