zdtm: fix a race condition during concurrent execution of tests

When tests are executed concurrently all of them use
the same root, so libraries must be copied atomically.

Without this patch we can get errors like this:
cp: cannot create regular file ‘/tmp/criu-root.m45u5Y/lib64/ld-linux-x86-64.so.2’: File exists
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.sh

@@ -223,6 +223,7 @@ construct_root()
 	local libdir=$root/lib
 	local libdir2=$root/lib64
 	local tmpdir=$root/tmp
+	local lname, tname
 
 	mkdir -p $root/bin
 	cp $ps_path $root/bin
@@ -242,13 +243,34 @@ construct_root()
 	#	/lib/ld-linux-armhf.so.3 (0xb6f0b000)
 
 	for i in `ldd $test_path $ps_path | grep -P '^\s' | grep -v vdso | sed "s/.*=> //" | awk '{ print $1 }'`; do
-		local lib=`basename $i`
-		[ -f $libdir/$lib ] && continue ||
-		[ -f $i ] && cp $i $libdir && cp $i $libdir2 && continue ||
-		[ -f /lib64/$i ] && cp /lib64/$i $libdir && cp /lib64/$i $libdir2 && continue ||
-		[ -f /usr/lib64/$i ] && cp /usr/lib64/$i $libdir && cp /usr/lib64/$i $libdir2 && continue ||
-		[ -f /lib/x86_64-linux-gnu/$i ] && cp /lib/x86_64-linux-gnu/$i $libdir && cp /lib/x86_64-linux-gnu/$i $libdir2 && continue ||
-		[ -f /lib/arm-linux-gnueabi/$i ] && cp /lib/arm-linux-gnueabi/$i $libdir && cp /lib/arm-linux-gnueabi/$i $libdir2 && continue || echo "Failed at " $i && return 1
+		local ldir, lib=`basename $i`
+
+		[ -f $libdir2/$lib ] && continue # fast path
+
+		if [ -f $i ]; then
+			lname=$i
+		elif [ -f /lib64/$i ]; then
+			lname=/lib64/$i
+		elif [ -f /usr/lib64/$i ]; then
+			lname=/usr/lib64/$i
+		elif [ -f /lib/x86_64-linux-gnu/$i ]; then
+			lname=/lib/x86_64-linux-gnu/$i
+		elif [ -f /lib/arm-linux-gnueabi/$i ]; then
+			lname=/lib/arm-linux-gnueabi/$i
+		else 
+			echo "Failed at " $i;
+			return 1
+		fi
+
+		# When tests are executed concurrently all of them use the same root,
+		# so libraries must be copied atomically.
+
+		for ldir in "$libdir" "$libdir2"; do
+			tname=$(mktemp $ldir/lib.XXXXXX)
+			cp -pf $lname $tname &&
+			mv -n $tname $ldir/$lib || return 1
+			[ -f $tname ] && unlink $tname
+		done
 	done
 
 	# make 'tmp' dir under new root