mptcp: fix uninit-value in mptcp_incoming_options

Added initialization use_ack to mptcp_parse_option().

Reported-by:  <syzbot+b834a6b2decad004cfa1@syzkaller.appspotmail.com>
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>