ceph-disk: protect deactivate with activate lock

ceph-disk: protect deactivate with activate lock

When ceph-disk prepares the disk, it triggers udev events and each of
them ceph-disk activate. If systemctl stop ceph-osd@2 happens while
there still are ceph-disk activate in flight, the systemctl stop may be
cancelled by the systemctl enable issued by one of the pending ceph-disk
activate.

This only matters in a test environment where disks are destroyed
shortly after they are activated.

Signed-off-by: Loic Dachary <loic@dachary.org>