tipc: add an extra conn_get in tipc_conn_alloc

tipc: add an extra conn_get in tipc_conn_alloc

One extra conn_get() is needed in tipc_conn_alloc(), as after
tipc_conn_alloc() is called, tipc_conn_close() may free this
con before deferencing it in tipc_topsrv_accept():

   tipc_conn_alloc();
   newsk = newsock->sk;
                                 <---- tipc_conn_close();
   write_lock_bh(&sk->sk_callback_lock);
   newsk->sk_data_ready = tipc_conn_data_ready;

Then an uaf issue can be triggered:

  BUG: KASAN: use-after-free in tipc_topsrv_accept+0x1e7/0x370 [tipc]
  Call Trace:
   <TASK>
   dump_stack_lvl+0x33/0x46
   print_report+0x178/0x4b0
   kasan_report+0x8c/0x100
   kasan_check_range+0x179/0x1e0
   tipc_topsrv_accept+0x1e7/0x370 [tipc]
   process_one_work+0x6a3/0x1030
   worker_thread+0x8a/0xdf0

This patch fixes it by holding it in tipc_conn_alloc(), then after
all accessing in tipc_topsrv_accept() releasing it. Note when does
this in tipc_topsrv_kern_subscr(), as tipc_conn_rcv_sub() returns
0 or -1 only, we don't need to check for "> 0".

Fixes: c5fa7b3cf3cb ("tipc: introduce new TIPC server infrastructure")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>