osd/scrub: reduce osd_requested_scrub_priority default value

osd/scrub: reduce osd_requested_scrub_priority default value

No scrub messages should have a higher priority than
client op messages. The default value should therefore be
below osd_client_op_priority, which is 63.

But as a followup PR would eliminate the special queue
priority for 'requested' scrub messages, the default value
is set to equal osd_scrub_priority.

Fixes: https://tracker.ceph.com/issues/68072
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>