osd/PG: decay scrub_chunk_max too if scrub is preempted

osd/PG: decay scrub_chunk_max too if scrub is preempted

In normal case we'll at least scrub as many objects as
osd_scrub_chunk_max specified at a time, so the current
backoff mechanism should have very limit effect.
Decay both osd_scrub_chunk_min and osd_scrub_chunk_max
should instead be a better resolution.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>
(cherry picked from commit a9260524676ac28742e5a945de93b87ae985017e)