sctp: initialize sin6_flowinfo for ipv6 addrs in sctp_inet6addr_event

syzbot reported a kernel-infoleak, which is caused by an uninitialized
field(sin6_flowinfo) of addr->a.v6 in sctp_inet6addr_event().
The call trace is as below:

  BUG: KMSAN: kernel-infoleak in _copy_to_user+0x19a/0x230 lib/usercopy.c:33
  CPU: 1 PID: 8164 Comm: syz-executor2 Not tainted 4.20.0-rc3+ #95
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
  Google 01/01/2011
  Call Trace:
    __dump_stack lib/dump_stack.c:77 [inline]
    dump_stack+0x32d/0x480 lib/dump_stack.c:113
    kmsan_report+0x12c/0x290 mm/kmsan/kmsan.c:683
    kmsan_internal_check_memory+0x32a/0xa50 mm/kmsan/kmsan.c:743
    kmsan_copy_to_user+0x78/0xd0 mm/kmsan/kmsan_hooks.c:634
    _copy_to_user+0x19a/0x230 lib/usercopy.c:33
    copy_to_user include/linux/uaccess.h:183 [inline]
    sctp_getsockopt_local_addrs net/sctp/socket.c:5998 [inline]
    sctp_getsockopt+0x15248/0x186f0 net/sctp/socket.c:7477
    sock_common_getsockopt+0x13f/0x180 net...