rcutorture: Add --datestamp parameter to kvm-again.sh

This commit adds a --datestamp parameter to kvm-again.sh, which, in
contrast to the existing --rundir argument, specifies only the last
segments of the pathname.  This addition enables torture.sh to use
kvm-again.sh in order to avoid redundant kernel builds.

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