mdadm: fix reshape from RAID5 to RAID6 with backup file

mdadm: fix reshape from RAID5 to RAID6 with backup file

Reshaping a 3-disk RAID5 to 4-disk RAID6 will cause a hang of
the resync after the grow.

Adding a spare disk to avoid degrading the array when growing
is successful, but not successful when supplying a backup file
on the command line. If the reshape job is not already running,
set the sync_max value to max.

Signed-off-by: Nigel Croxon <ncroxon@redhat.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>