tipc: fix an information leak in tipc_topsrv_kern_subscr

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

tipc: fix an information leak in tipc_topsrv_kern_subscr

Use a 8-byte write to initialize sub.usr_handle in
tipc_topsrv_kern_subscr(), otherwise four bytes remain uninitialized
when issuing setsockopt(..., SOL_TIPC, ...).
This resulted in an infoleak reported by KMSAN when the packet was
received:

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BUG: KMSAN: kernel-infoleak in copyout+0xbc/0x100 lib/iov_iter.c:169
instrument_copy_to_user ./include/linux/instrumented.h:121
copyout+0xbc/0x100 lib/iov_iter.c:169
_copy_to_iter+0x5c0/0x20a0 lib/iov_iter.c:527
copy_to_iter ./include/linux/uio.h:176
simple_copy_to_iter+0x64/0xa0 net/core/datagram.c:513
__skb_datagram_iter+0x123/0xdc0 net/core/datagram.c:419
skb_copy_datagram_iter+0x58/0x200 net/core/datagram.c:527
skb_copy_datagram_msg ./include/linux/skbuff.h:3903
packet_recvmsg+0x521/0x1e70 net/packet/af_packet.c:3469
____sys_recvmsg+0x2c4/0x810 net/socket.c:?
___sys_recvmsg+0x217/0x840 net/socket.c:2743
__sys_recvmsg net/socket.c:2773
__do_sys_recvmsg net/socket.c:2783
__se_sys_recvmsg net/socket.c:2780
__x64_sys_recvmsg+0x364/0x540 net/socket.c:2780
do_syscall_x64 arch/x86/entry/common.c:50
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd arch/x86/entry/entry_64.S:120

...

Uninit was stored to memory at:
tipc_sub_subscribe+0x42d/0xb50 net/tipc/subscr.c:156
tipc_conn_rcv_sub+0x246/0x620 net/tipc/topsrv.c:375
tipc_topsrv_kern_subscr+0x2e8/0x400 net/tipc/topsrv.c:579
tipc_group_create+0x4e7/0x7d0 net/tipc/group.c:190
tipc_sk_join+0x2a8/0x770 net/tipc/socket.c:3084
tipc_setsockopt+0xae5/0xe40 net/tipc/socket.c:3201
__sys_setsockopt+0x87f/0xdc0 net/socket.c:2252
__do_sys_setsockopt net/socket.c:2263
__se_sys_setsockopt net/socket.c:2260
__x64_sys_setsockopt+0xe0/0x160 net/socket.c:2260
do_syscall_x64 arch/x86/entry/common.c:50
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd arch/x86/entry/entry_64.S:120

Local variable sub created at:
tipc_topsrv_kern_subscr+0x57/0x400 net/tipc/topsrv.c:562
tipc_group_create+0x4e7/0x7d0 net/tipc/group.c:190

Bytes 84-87 of 88 are uninitialized
Memory access of size 88 starts at ffff88801ed57cd0
Data copied to user address 0000000020000400
...
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