osd/PeeringState.h: ignore RemoteBackfillReserved in WaitLocalBackfillReserved
It is possible to dequeue an outstanding RemoteBackfillReserved, though we may have
already released reservations for that backfill target. Currently, if this happens
while we are in WaitLocalBackfillReserved, it can lead to a crash on the primary.
Prevent this by treating this condition as a no-op.
The longer term fix is to add a RELEASE_ACK mechanism, which prevents the primary
from scheduling a backfill retry until all the RELEASE_ACKs have been received.