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

In the Linux kernel, the following vulnerability has been resolved:

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.