osd: support osd_scrub_extended_sleep

osd: support osd_scrub_extended_sleep

1. always take osd_scrub_sleep for manually initiated
   scrubs
2. when scrub_time_permit() return true for scheduled
   ones, the existing osd_scrub_sleep is used
3. when scrub_time_permit() return false for scheduled
   ones, there may be 2 scenarios
   3.1 if osd_scrub_extended_sleep <= osd_scrub_sleep,
       let's take osd_scrub_sleep
   3.2 otherwise, let's take osd_scrub_extended_sleep

Fixes: http://tracker.ceph.com/issues/40955
Signed-off-by: Jeegn Chen <jeegnchen@tencent.com>