torture: Fix kvm.sh --datestamp regex check

Some versions of grep are happy to interpret a nonsensically placed "-"
within a "[]" pattern as a dash, while others give an error message.
This commit therefore places the "-" at the end of the expression where
it was supposed to be in the first place.

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

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

@@ -120,7 +120,7 @@ do
 		shift
 		;;
 	--datestamp)
-		checkarg --datestamp "(relative pathname)" "$#" "$2" '^[a-zA-Z0-9._-/]*$' '^--'
+		checkarg --datestamp "(relative pathname)" "$#" "$2" '^[a-zA-Z0-9._/-]*$' '^--'
 		ds=$2
 		shift
 		;;