dm raid: correct resizing state relative to reshape space in ctr

Pay attention to existing reshape space to define if a raid set needs
resizing.  Otherwise we can hit "Can't resize a reshaping raid set"
when a reshape is being requested.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>