scftorture: Increase memory provided to guest OS

The tradition, extending back almost a full year, has been 2GB plus an
additional number of GBs equal to the number of CPUs divided by sixteen.
This tradition has served scftorture well, even the CONFIG_PREEMPT=y
version running KASAN within guest OSes having 40 CPUs.  However, this
test recently started OOMing on larger systems, and this commit therefore
gives this test an additional GB of memory.

It is quite possible that further testing on larger systems will show
a need to decrease the divisor from 16 to (say) 8, but that is a change
to make once it has been demonstrated to be required.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>