mgr: fix race between module load and notify

mgr: fix race between module load and notify

When starting a module, there was a time window between we
registered the module in the module list and loaded it in the
finisher thread, when a notify could be delivered to not fully
initialized module.

We can avoid this by delaying registering the module in the
module list until it is successfully initialized.

Fixes: https://tracker.ceph.com/issues/41736
Signed-off-by: Mykola Golub <mgolub@suse.com>