mon/PGMonitor: set tid on no-op PGStatsAck

The OSD needs to know the tid.  Both generally, and specifically because
the flush_pg_stats may be blocking on it.

Fixes: #8280
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>
(cherry picked from commit 5a6ae2a978dcaf96ef89de3aaa74fe951a64def6)