osd/scrub: remove reliance of Scrubber objects' logging on the PG

osd/scrub: remove reliance of Scrubber objects' logging on the PG

Modify the Scrubber's sub-objects to use their own gen_prefix()
functions, instead of using PG::gen_prefix().

Fixes: https://tracker.ceph.com/issues/51843
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>
(cherry picked from commit 2aeb9263d643b19d59219e8e187e1a0fa0292693)

Conflicts:
        src/osd/PG.h
        src/osd/PrimaryLogScrub.cc
        src/osd/pg_scrubber.cc
        src/osd/pg_scrubber.h

Conflict resolution:
- manually removing some scrub scheduling changes from
  PR #40984
- pg_scrubber.h: removing some irrelevant lines that were dragged
  in.
- PG.h: restoring lines removed by the merge.