mptcp: add support for implicit flag

Kernel supports implicit flag since commit d045b9eb95a9 ("mptcp:
introduce implicit endpoints"), included in v5.18.

Let's add support for displaying it to iproute2.

Before this change:
$ ip mptcp endpoint show
10.0.2.2 id 1 rawflags 10

After this change:
$ ip mptcp endpoint show
10.0.2.2 id 1 implicit

Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>
2 files changed