blk-throttle: set REQ_THROTTLED from throtl_charge_bio() and gate stats update with it

With proper hierarchy support, a bio can be dispatched multiple times
until it reaches the top-level service_queue and we don't want to
update dispatch stats at each step.  They are local stats and will be
kept local.  If recursive stats are necessary, they should be
implemented separately and definitely not by updating counters
recursively on each dispatch.

This patch moves REQ_THROTTLED setting to throtl_charge_bio() and gate
stats update with it so that dispatch stats are updated only on the
first time the bio is charged to a throtl_grp, which will always be
the throtl_grp the bio was originally queued to.

This means that REQ_THROTTLED would be set even for bios which don't
get throttled.  As we don't want bios to leave blk-throtl with the
flag set, move REQ_THROTLLED clearing to the end of blk_throtl_bio()
and clear if the bio is being issued directly.

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