rcutorture: Make kvm.sh allow more memory for --kasan runs

KASAN allots significant memory to track allocation state, and the amount
of memory has increased recently, which results in frequent OOMs on a
few of the rcutorture scenarios.  This commit therefore provides 2G of
memory for --kasan runs, up from the 512M default.

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