rcutorture/kvm.sh: Support execution from any directory

The 'kvm.sh' rcutorture script requires that it be invoked from the top
of Linux-kernel source tree.  It is just a subtle restriction, but users
using it for the first time could forget the restriction and be confused.
Moreover, it makes commands a little longer, which can be frustrating.
This commit therefore lets users invoke the script from any location.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>