Improve allocation and use of space for bitmaps in version1 metadata

Improve allocation and use of space for bitmaps in version1 metadata

Depending on the size of the array we reserve space for up to 128K
of bitmap, and we use it where possible.
When hot-adding to a version 1.0 we can still only use the 3K at the
end though - need a sysfs interface to improve that.

If a small chunksize is requested on Create, we don't auto-enlarge
the reserved space - this still needs to be fixed.