selftests: mptcp: pm: rm the right tmp file

"$err" is a variable pointing to a temp file. "$out" is not: only used
as a local variable in "check()" and representing the output of a
command line.

Fixes: eedbc68532 (selftests: add PM netlink functional tests)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/mptcp/pm_netlink.sh

@@ -30,7 +30,7 @@ ret=0
 
 cleanup()
 {
-	rm -f $out
+	rm -f $err
 	ip netns del $ns1
 }