mptcp: Change mailing list address

The mailing list for MPTCP maintenance has moved to the
kernel.org-supported mptcp@lists.linux.dev address.

Complete, combined archives for both lists are now hosted at
https://lore.kernel.org/mptcp



Cc: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>