osd: shut down the MgrClient before osd_fast_shutdown

osd: shut down the MgrClient before osd_fast_shutdown

We should shut down the MgrClient before osd_fast_shutdown.

This approach avoids a race condition that sometimes occurs
when the MgrClient attempts to gather `pg_stats` from the database
after it has been unmounted, or when it is in the process of being
unmounted.

Fixes: https://tracker.ceph.com/issues/56101
Signed-off-by: Laura Flores <lflores@redhat.com>
Signed-off-by: Brad Hubbard <bhubbard@redhat.com>