rcutorture: Add --kill-previous option to terminate previous kvm.sh runs

rcutorture: Add --kill-previous option to terminate previous kvm.sh runs

When kvm.sh is killed, its child processes (make, gcc, qemu, etc.) may
continue running. This prevents new kvm.sh instances from starting even
though the parent is gone.

Add a --kill-previous option that uses fuser(1) to terminate all
processes holding the flock file before attempting to acquire it. This
provides a clean way to recover from stale/zombie kvm.sh runs which
sometimes may have lots of qemu and compiler processes still disturbing.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>