osd/PG: move reject_reservation out of RemoteReservationRejected reaction

osd/PG: move reject_reservation out of RemoteReservationRejected reaction

The RemoteReservationRejected event is also submitted when we are a
replica or backfill target and get a MBackfillReserve REJECT message
because the primary canceled or was preempted.  In that case, we don't
want to send a REJECT back to the primary; we only need to send it in the
cases where *we*, locally, decide to reject.  Move the call to those call
sites.

Signed-off-by: Sage Weil <sage@redhat.com>