selftests: mptcp: clean tmp files in simult_flows

'$cin' and '$sin' variables are local to a function: they are then not
available from the cleanup trap.

Instead, we need to use '$large' and '$small' that are not local and
defined just before setting the trap.

Without this patch, running this script in a loop might cause a:

  write: No space left on device

issue.

Fixes: 1a418cb8e8 ("mptcp: simult flow self-tests")
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -22,8 +22,8 @@ usage() {
 
 cleanup()
 {
-	rm -f "$cin" "$cout"
-	rm -f "$sin" "$sout"
+	rm -f "$cout" "$sout"
+	rm -f "$large" "$small"
 	rm -f "$capout"
 
 	local netns