Merge pull request #69824 from mohit84/backfill_stuck

crimson/osd: fix backfill deadlock in BackfillState::Waiting

BackfillState::Enqueuing unconditionally posts RequestWaiting{}
when budget available return false. Waiting can exit only via
ObjectPushed which requires an in-flight push. If the budget
is exhausted with no pushes in flight no objectPush will ever
arrive and the PG stalls permanently.

Solution: Introduce a dedicated BudgetBlocked state. When
budget_available() is false and tracked_objects_completed()==true
post RequestBudgetBlocked{} instead of RequestWaiting{}.
BudgetBlocked calls get_backfill_throttle() to suspend until a slots
free up, then fires BudgetAvailable{} which transitions directly back
to Enqueuing to dispatch the next batch of pushes.

Fixes: https://tracker.ceph.com/issues/77804

diff --cc src/crimson/osd/pg_recovery.cc
Simple merge