Add updating component_size to manager thread of mdmon

Add updating component_size to manager thread of mdmon

Mdmon does not update component_size now. It is wrong because in case
of size's expansion component_size is changed by mdadm but mdmon does not
reread its new value and uses a wrong, old one. As a result the metadata
is incorrect during size's expansion. It contains no information that
resync is in progress (there is no checkpoint too). The metadata is
as if resync has already been finished but it has not.

Component_size will be set to match information in sysfs. This value
will be updated by manager thread in manage_member() function.
Now mdmon uses the correct, current value of component_size and the
correct metadata (containing information about resync and checkpoint)
is written.

Signed-off-by: Pawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>