torture: Make kvm-check-branches.sh use --allcpus

Currently the kvm-check-branches.sh script calculates the number of CPUs
and passes this to the kvm.sh --cpus command-line argument.  This works,
but this commit saves a line by instead using the new kvm.sh --allcpus
command-line argument.

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