selftests: mptcp: clean tmp files in simult_flows

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: 3896873079e5 ("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>