FIX: External metadata sometimes is not updated

FIX: External metadata sometimes is not updated

External metadata sometimes is not updated.
It can be observed during 2 raid0 arrays Capacity Expansion.
New array size is not set, because metadata is not updated and on the reshape
end mdadm doesn't read new array size from metadata.
This happens when mdmon finishes his work (due to takeover to raid0),
before all metadata updates are processed.

Make sure that all updates are flushed to disk before executing takeover.

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