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.