mdadm: change the num of cluster node

mdadm: change the num of cluster node

This extends nodes option for assemble mode, make the num of
cluster node could be change by user.

Before that, it is necessary to ensure there are enough space
for those nodes, calc_bitmap_size is introduced to calculate
the bitmap size of each node.

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: NeilBrown <neilb@suse.de>