generic/072: limit max cpu number to 8

generic/072: limit max cpu number to 8

Right now generic/072 scales the loop count based on the cpu count. But
on hosts with many cpus(100+), generic/072 runs for hours and generates
very high system load.

Given that the original bug can be reproduced easily on unpatched
kernel, the great number of loops and long run time are not needed. So
limiting the cpu number to 8 (which gives around 20 seconds run time on
my test vm with 8 vcpus) seems reasonable.

Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>