Assemble: when forcing a single-degraded RAID6 array, trigger a 'repair'.

Assemble: when forcing a single-degraded RAID6 array, trigger a 'repair'.

When an active/degraded RAID6 array is force-started we clear the
'active' flag, but it is still possible that some parity is
no in sync.  This is because there are two parity block.
It would be nice to be able to tell the kernel "P is OK, Q maybe not".
But that is not possible.

So when we force-assemble such an array, trigger a 'repair' to fix up
any errant Q blocks.

This is not ideal as a restart during the repair will not be continued
after the restart, but it is the best we can do without kernel help.

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