imsm: return free space after volume for expand

imsm: return free space after volume for expand

merge_extends() routine searches for the biggest free space. For expand,
it works only in standard cases where the last volume is expanded and
the free space is determined after the last volume.
Add volume index to extent struct and use that do determine size after
super->current_vol during expand.

Limitation to last volume is no longer needed. It unblocks scenarios
where kill-subarray is used to remove first volume and later it is
recreated (now it is the second volume, even if it is placed before
existing one).

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>