osd/scrub: exempt only operator scrubs from max_scrubs limit

osd/scrub: exempt only operator scrubs from max_scrubs limit

Existing code exempts all 'high priority' scrubs, including for example
'after_repair' and 'mandatory on invalid history' scrubs from the limit.

PGs that do not have valid last-scrub data (which is what we have when
a pool is first created) - are set to shallow-scrub immediately.
Unfortunately - this type of scrub is (in the low granularity implemented
in existing code) is 'high priority'.
Which means that a newly created pool will have all its PGs start
scrubbing, regardless of concurrency (or any other) limits.

Fixes: https://tracker.ceph.com/issues/67253
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>
(cherry picked from commit babd65e412266f5c734f7a2b57d87657d3470c47)
conflict resolution:
- eliminating irrelevant 'main' code that was picked into this branch.
- the code to set the scrub_job's flag moved to osd_scrub_sched.cc,
  where the corresponding function is.