torture: Scale scftorture memory based on number of CPUs

As the number of CPUs increases, the number of outstanding no-wait
smp_call_function() handlers also increases, so that the default of
2G of memory is not always sufficient on 80-CPU systems.  This commit
therefore scales the amount of memory specified to qemu based on the
number of CPUs specified to the scftorture test instance.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>