Convert a bitmap=none device to clustered

Convert a bitmap=none device to clustered

This adds the ability to convert a regular md without bitmap
(--bitmap=none) to a clustered device (--bitmap=clustered).

To convert a device with --bitmap=internal or --bitmap=external,
you have to convert to --bitmap=none and then re-execute the
command with --bitmap=clustered.

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