Flush mdmon before next reshape step during container operation

Flush mdmon before next reshape step during container operation

Using takeover operation for grow purposes, mdadm has to be sure
that mdmon processes all updates, and if necessary it will be closed
at takeover to raid0 operation. If mdmon is late, next array in container
is processed and due to race condition mdmon closes itself instead to monitor
next reshape operation.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>