ReplicatedPG::promote_object: check scrubber and block if necessary

ReplicatedPG::promote_object: check scrubber and block if necessary

Otherwise, we might attempt to promote into an in-progress scrub
interval causing 11156.  I would have added a return value to
promote_object(), but could not find an existing user which
cared to distinguish the cases, even with a null op passed.
All existing users are in maybe_handle_cache.  The ones which
pass a null op are for promoting the object in parallel
with a proxy -- a case where not actually performing the promote
does not really matter.

Fixes: #11156
Signed-off-by: Samuel Just <sjust@redhat.com>