tipc: fix a null pointer deref on error path

In tipc_topsrv_kern_subscr() when s->tipc_conn_new() fails
we call tipc_close_conn() to clean up, but in this case
calling conn_put() is just enough.

This fixes the folllowing crash:

 kasan: GPF could be caused by NULL-ptr deref or user memory access
 general protection fault: 0000 [#1] SMP KASAN
 Dumping ftrace buffer:
    (ftrace buffer empty)
 Modules linked in:
 CPU: 0 PID: 3085 Comm: syzkaller064164 Not tainted 4.15.0-rc1+ #137
 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 task: 00000000c24413a5 task.stack: 000000005e8160b5
 RIP: 0010:__lock_acquire+0xd55/0x47f0 kernel/locking/lockdep.c:3378
 RSP: 0018:ffff8801cb5474a8 EFLAGS: 00010002
 RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
 RDX: 0000000000000004 RSI: 0000000000000000 RDI: ffffffff85ecb400
 RBP: ffff8801cb547830 R08: 0000000000000001 R09: 0000000000000000
 R10: 0000000000000000 R11: ffffffff87489d60 R12: ffff8801cd2980c0
 R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000000020
 FS:  00000000014ee880(0000) GS:ffff8801db400000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007ffee2426e40 CR3: 00000001cb85a000 CR4: 00000000001406f0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:4004
  __raw_spin_lock_bh include/linux/spinlock_api_smp.h:135 [inline]
  _raw_spin_lock_bh+0x31/0x40 kernel/locking/spinlock.c:175
  spin_lock_bh include/linux/spinlock.h:320 [inline]
  tipc_subscrb_subscrp_delete+0x8f/0x470 net/tipc/subscr.c:201
  tipc_subscrb_delete net/tipc/subscr.c:238 [inline]
  tipc_subscrb_release_cb+0x17/0x30 net/tipc/subscr.c:316
  tipc_close_conn+0x171/0x270 net/tipc/server.c:204
  tipc_topsrv_kern_subscr+0x724/0x810 net/tipc/server.c:514
  tipc_group_create+0x702/0x9c0 net/tipc/group.c:184
  tipc_sk_join net/tipc/socket.c:2747 [inline]
  tipc_setsockopt+0x249/0xc10 net/tipc/socket.c:2861
  SYSC_setsockopt net/socket.c:1851 [inline]
  SyS_setsockopt+0x189/0x360 net/socket.c:1830
  entry_SYSCALL_64_fastpath+0x1f/0x96

Fixes: 14c04493 ("tipc: add ability to order and receive topology events in driver")
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Jon Maloy <jon.maloy@ericsson.com>
Cc: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>