crimson/osd: acquire throttle when scanning replica/primary for backfill

crimson/osd: acquire throttle when scanning replica/primary for backfill

The backfill state machine called budget_available() before deciding to
scan, but request_primary_scan() and request_replica_scan() never
actually acquired the throttle slot.  This meant scans could proceed
without any resource reservation, defeating the QoS intent of the
throttler introduced in 791772f1c0.

In this change, we fix this by acquiring the throttle before initiating
each scan.

Fixes: https://tracker.ceph.com/issues/70808
Signed-off-by: Kefu Chai <k.chai@proxmox.com>