torture: Add --allcpus argument to the kvm.sh script

Leaving off the kvm.sh script's --cpus argument results in the script
testing the scenarios sequentially, which can be quite slow.  However,
having to specify the actual number of CPUs can be error-prone.
This commit therefore adds a --allcpus argument that causes kvm.sh to
use all available CPUs.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/rcutorture/bin/kvm.sh

@@ -73,6 +73,10 @@ usage () {
 while test $# -gt 0
 do
 	case "$1" in
+	--allcpus)
+		cpus=$TORTURE_ALLOTED_CPUS
+		max_cpus=$TORTURE_ALLOTED_CPUS
+		;;
 	--bootargs|--bootarg)
 		checkarg --bootargs "(list of kernel boot arguments)" "$#" "$2" '.*' '^--'
 		TORTURE_BOOTARGS="$2"