Allow a metadata update to have a linked list of allocated spaces.

Allow a metadata update to have a linked list of allocated spaces.

Sometimes one metadata update will require allocating several
larger data structures.  As 'monitor' cannot allocate, 'manager'
must, so it must be able to attach a list of allocates to the
update, and importantly it must be able to easily free them.

So add a 'space_list' element to metadata updates where each
element on the list starts with a pointer to the next.

Signed-off-by: NeilBrown <neilb@suse.de>