torture: Allow kvm.sh --datestamp to specify subdirectories

Scripts like kvm-check-branches.sh group runs under a single directory
in resdir in order to allow easier retrospective analysis.  However, they
do this by letting kvm.sh create a directory as usual and then moving it
after the run.  This can be very confusing when looking at the results
while kvm-check-branches.sh is running.  This commit therefore enables
--datestamp to hand subdirectories to kvm.sh.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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

@@ -113,7 +113,7 @@ do
 		shift
 		;;
 	--datestamp)
-		checkarg --datestamp "(relative pathname)" "$#" "$2" '^[^/]*$' '^--'
+		checkarg --datestamp "(relative pathname)" "$#" "$2" '^[a-zA-Z0-9._-/]*$' '^--'
 		ds=$2
 		shift
 		;;
@@ -375,7 +375,7 @@ if ! test -e $resdir
 then
 	mkdir -p "$resdir" || :
 fi
-mkdir $resdir/$ds
+mkdir -p $resdir/$ds
 TORTURE_RESDIR="$resdir/$ds"; export TORTURE_RESDIR
 TORTURE_STOPFILE="$resdir/$ds/STOP"; export TORTURE_STOPFILE
 echo Results directory: $resdir/$ds