ipv6: addrlabel: fix infoleak when sending struct ifaddrlblmsg to network

When copying a `struct ifaddrlblmsg` to the network, __ifal_reserved
remained uninitialized, resulting in a 1-byte infoleak:

  BUG: KMSAN: kernel-network-infoleak in __netdev_start_xmit ./include/linux/netdevice.h:4841
   __netdev_start_xmit ./include/linux/netdevice.h:4841
   netdev_start_xmit ./include/linux/netdevice.h:4857
   xmit_one net/core/dev.c:3590
   dev_hard_start_xmit+0x1dc/0x800 net/core/dev.c:3606
   __dev_queue_xmit+0x17e8/0x4350 net/core/dev.c:4256
   dev_queue_xmit ./include/linux/netdevice.h:3009
   __netlink_deliver_tap_skb net/netlink/af_netlink.c:307
   __netlink_deliver_tap+0x728/0xad0 net/netlink/af_netlink.c:325
   netlink_deliver_tap net/netlink/af_netlink.c:338
   __netlink_sendskb net/netlink/af_netlink.c:1263
   netlink_sendskb+0x1d9/0x200 net/netlink/af_netlink.c:1272
   netlink_unicast+0x56d/0xf50 net/netlink/af_netlink.c:1360
   nlmsg_unicast ./include/net/netlink.h:1061
   rtnl_unicast+0x5a/0x80 net/core/rtnetlink.c:758
   ip6addrlbl_get+0xfad/0x10f0 net/ipv6/addrlabel.c:628
   rtnetlink_rcv_msg+0xb33/0x1570 net/core/rtnetlink.c:6082
  ...
  Uninit was created at:
   slab_post_alloc_hook+0x118/0xb00 mm/slab.h:742
   slab_alloc_node mm/slub.c:3398
   __kmem_cache_alloc_node+0x4f2/0x930 mm/slub.c:3437
   __do_kmalloc_node mm/slab_common.c:954
   __kmalloc_node_track_caller+0x117/0x3d0 mm/slab_common.c:975
   kmalloc_reserve net/core/skbuff.c:437
   __alloc_skb+0x27a/0xab0 net/core/skbuff.c:509
   alloc_skb ./include/linux/skbuff.h:1267
   nlmsg_new ./include/net/netlink.h:964
   ip6addrlbl_get+0x490/0x10f0 net/ipv6/addrlabel.c:608
   rtnetlink_rcv_msg+0xb33/0x1570 net/core/rtnetlink.c:6082
   netlink_rcv_skb+0x299/0x550 net/netlink/af_netlink.c:2540
   rtnetlink_rcv+0x26/0x30 net/core/rtnetlink.c:6109
   netlink_unicast_kernel net/netlink/af_netlink.c:1319
   netlink_unicast+0x9ab/0xf50 net/netlink/af_netlink.c:1345
   netlink_sendmsg+0xebc/0x10f0 net/netlink/af_netlink.c:1921
  ...

This patch ensures that the reserved field is always initialized.

Reported-by:  <syzbot+3553517af6020c4f2813f1003fe76ef3cbffe98d@syzkaller.appspotmail.com>
Fixes: 2a8cc6c8 ("[IPV6] ADDRCONF: Support RFC3484 configurable address selection policy table.")
Signed-off-by: Alexander Potapenko <glider@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>