repair: limit auto-striding concurrency apprpriately

repair: limit auto-striding concurrency apprpriately

It's possible to have filesystems with hundreds of AGs on systems
with little concurrency and resources. In this case, we can easily
exhaust memory and fail to create threads and have all sorts of
interesting problems.

xfs/250 can cause this to occur, with failures like:

        - agno = 707
        - agno = 692
fatal error -- cannot create worker threads, error = [11] Resource temporarily unavailable

And this:

        - agno = 484
        - agno = 782
failed to create prefetch thread: Resource temporarily unavailable

Because it's trying to create more threads than a poor little 512MB
single CPU ia32 box can handle.

So, limit concurrency to a maximum of numcpus * 8 to prevent this.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>