fix: correct metadata's update communication

fix: correct metadata's update communication

The problem occurs when array under migration is assembled incrementally.
st->update_tail is not initialized in function
assemble_container_content() and during reshape
the checkpoint information in metadata is not being updated.

The value of st->update_tail is now initialized in function
assemble_container_content() and during reshape the checkpoint
information in metadata is being updated correctly on all disks.

Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>