Fix serious memory leak

Fix serious memory leak

During reshape function restore_stripes is called periodically
and every time the buffer stripe_buf (of size raid_disks*chunk_size)
is allocated but is not freed. It happens also upon successful completion.
In case of huge arrays it can lead to the seizure of the entire
system memory (even of the order of gigabytes).

Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>