md: notify about new spare disk in the container

In case of external metadata arrays spare disks are added to containers
first. mdadm keeps monitoring /proc/mdstat output and when spare disk is
available, it moves it from the container to the array. The problem is
there is no notification of new spare disk in the container and mdadm
waits a long time (until timeout) before it takes the action.

Signed-off-by: Alexey Obitotskiy <aleksey.obitotskiy@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>