rcutorture: Prevent concurrent kvm.sh runs on same source tree

rcutorture: Prevent concurrent kvm.sh runs on same source tree

Add flock-based locking to kvm.sh to prevent multiple instances from
running concurrently on the same source tree. This prevents build
failures caused by one instance's "make clean" deleting generated files
while another instance is building causing build failures.

The lock file is placed in the rcutorture directory and added to
.gitignore.

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>