Create n bitmaps for clustered mode

Create n bitmaps for clustered mode

For a clustered MD, create bitmaps equal to number of nodes so
each node has an independent bitmap.

Only the first bitmap is has the bits set so that the first node
that assembles the device also performs the sync.

The bitmaps are aligned to 4k boundaries.

On-disk format:

0                    4k                     8k                    12k
-------------------------------------------------------------------
| idle                | md super            | bm super [0] + bits |
| bm bits[0, contd]   | bm super[1] + bits  | bm bits[1, contd]   |
| bm super[2] + bits  | bm bits [2, contd]  | bm super[3] + bits  |
| bm bits [3, contd]  |                     |                     |

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