librbd: update hidden global config when setting pool config override

librbd: update hidden global config when setting pool config override

The new "dev"-level global config setting will be updated when any
pool-level config override is updated. librbd clients will detect
the new global-level config update and trigger a refresh. This avoids
the need for potentially tens of thousands of librbd clients
registering a watch on the pool metadata object or periodically polling
the pool metadata object for updates.

Fixes: https://tracker.ceph.com/issues/46694
Signed-off-by: Jason Dillaman <dillaman@redhat.com>