rcutorture: Make torture.sh allow for --kasan

The torture.sh script provides extra memory for scftorture and rcuscale.
However, the total memory provided is only 1G, which is less than the
2G that is required for KASAN testing.  This commit therefore ups the
torture.sh script's 1G to 2G.

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