mptcp: Set mtu_reduced in mptcp-specific inet_connection_sock_af_ops

Since 4fab9071950 (tcp: fix tcp_release_cb() to dispatch via address
family for mtu_reduced()) - backported to 3.14.22, mtu_reduced is no
more a sk_prot-field but part of inet_connection_sock_af_ops. In MPTCP
we have our own inet_connection_sock_af_ops, so we need to adapt
mtu_reduced as well.

Cc: Baptiste Jonglez <baptiste@bitsofnetworks.org>
Reported-by: Baptiste Jonglez <baptiste@bitsofnetworks.org>
Fixes: 4fab9071950 (tcp: fix tcp_release_cb() to dispatch via address family for mtu_reduced())
Signed-off-by: Christoph Paasch <christoph.paasch@gmail.com>
Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>