rcutorture: Avoid redundant builds for rcuscale and refscale in torture.sh

This commit causes torture.sh to use the new --bootargs and --datestamp
parameters to kvm-again.sh in order to avoid redundant kernel builds
during rcuscale and refscale testing.  This trims the better part of an
hour off of torture.sh runs that use --do-kasan.

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