selftests: mptcp: join: fix 'delete and re-add' test

selftests: mptcp: join: fix 'delete and re-add' test

mptcp_join 'delete and re-add' test fails when using ip mptcp:

  $ ./mptcp_join.sh -iI
  <snip>
  002 delete and re-add                    before delete[ ok ]
                                           mptcp_info subflows=1         [ ok ]
  Error: argument "ADDRESS" is wrong: invalid for non-zero id address
                                           after delete[fail] got 2:2 subflows expected 1

This happens because endpoint delete includes an ip address while id is
not 0, contrary to what is indicated in the ip mptcp man page:

"When used with the delete id operation, an IFADDR is only included when
the ID is 0."

This fixes the issue using the $addr variable in pm_nl_del_endpoint()
only when id is 0.

Fixes: 34aa6e3bccd8 ("selftests: mptcp: add ip mptcp wrappers")
Cc: stable@vger.kernel.org
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20230803-upstream-net-20230803-misc-fixes-6-5-v1-1-6671b1ab11cc@tessares.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>