mptcp: show all endpoints when no ID is specified

mptcp: show all endpoints when no ID is specified

According to 'ip mptcp help', 'endpoint show' can accept no argument:

  ip mptcp endpoint show [ id ID ]

It makes sense to print all endpoints when no filter is used.

So here if the following command is used, all endpoints are printed:

  ip mptcp endpoint show

Same as:

  ip mptcp endpoint

Fixes: 7e0767cd ("add support for mptcp netlink interface")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>