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")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
---
 tools/testing/selftests/net/mptcp/mptcp_join.sh | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
index 0ae8cafde439..b972feb9a3a2 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
@@ -678,6 +678,7 @@ pm_nl_del_endpoint()
 	local addr=$3
 
 	if [ $ip_mptcp -eq 1 ]; then
+		[ $id -ne 0 ] && addr=''
 		ip -n $ns mptcp endpoint delete id $id $addr
 	else
 		ip netns exec $ns ./pm_nl_ctl del $id $addr
-- 
2.41.0