imsm: improve getinfo_super when reshape is in progress.

imsm: improve getinfo_super when reshape is in progress.

If a reshape (migration) is happening, we might need to modify the information
with provide to md so that it can cope with the reshape.
For example, if a migration from 4-device RAID0 to 5-device RAID0 is
happening, we need to tell md that it is reshape from degraded
5-device RAID4 to degraded 6-device RAID4 so md doesn't handle direct
reshape of RAID0.

There may be other migrations supported by IMSM that need special
treatment here.

Signed-off-by: NeilBrown <neilb@suse.de>