tgkill: Fix argument validatation

Formerly, if tgid is specified as -1, tgkill() was equivalent to tkill().
Now it is treated as an error EINVAL.

Change-Id: I47bc75d439662a36dc6167c4446a5277422de507
Refs: 1380

test/issues/1380/C1380.sh

@@ -0,0 +1,26 @@
+#/bin/sh
+
+USELTP=1
+USEOSTEST=0
+
+. ../../common.sh
+
+issue="1380"
+tid=01
+
+for tp in tgkill03 tgkill01 tkill01 tkill02
+do
+	tname=`printf "C${issue}T%02d" ${tid}`
+	echo "*** ${tname} start *******************************"
+	sudo $MCEXEC $LTPBIN/$tp 2>&1 | tee $tp.txt
+	ok=`grep PASS $tp.txt | wc -l`
+	ng=`grep FAIL $tp.txt | wc -l`
+	if [ $ng = 0 ]; then
+		echo "*** ${tname} PASSED ($ok)"
+	else
+		echo "*** ${tname} FAILED (ok=$ok ng=$ng)"
+	fi
+	let tid++
+	echo ""
+done
+