selftests: mptcp: capture netlink events

This change adds to self-testing support for the MPTCP netlink interface
by capturing various MPTCP netlink events (and all their metadata)
associated with connections, subflows and address announcements.
It is used in self-testing scripts that exercise MPTCP netlink commands
to precisely validate those operations by examining the dispatched
MPTCP netlink events in response to those commands.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Kishen Maloor <kishen.maloor@intel.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>