tipc: fix use-after-free in tipc_sk_filter_rcv

tipc: fix use-after-free in tipc_sk_filter_rcv

skb free-ed in:
  1/ condition 1: tipc_sk_filter_rcv -> tipc_sk_proto_rcv
  2/ condition 2: tipc_sk_filter_rcv -> tipc_group_filter_msg
This leads to a "use-after-free" access in the next condition.

We fix this by intializing the variable at declaration, then it is safe
to check this variable to continue processing if condition matches.

syzbot report:

==================================================================
BUG: KASAN: use-after-free in tipc_sk_filter_rcv+0x2166/0x34f0
 net/tipc/socket.c:2167
Read of size 4 at addr ffff88808ea58534 by task kworker/u4:0/7

CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 5.0.0+ #61
Hardware name: Google Google Compute Engine/Google Compute Engine,
 BIOS Google 01/01/2011
Workqueue: tipc_send tipc_conn_send_work
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:187
 kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/generic_report.c:131
 tipc_sk_filter_rcv+0x2166/0x34f0 net/tipc/socket.c:2167
 tipc_sk_enqueue net/tipc/socket.c:2254 [inline]
 tipc_sk_rcv+0xc45/0x25a0 net/tipc/socket.c:2305
 tipc_topsrv_kern_evt+0x3b7/0x580 net/tipc/topsrv.c:610
 tipc_conn_send_to_sock+0x43e/0x5f0 net/tipc/topsrv.c:283
 tipc_conn_send_work+0x65/0x80 net/tipc/topsrv.c:303
 process_one_work+0x98e/0x1790 kernel/workqueue.c:2269
 worker_thread+0x98/0xe40 kernel/workqueue.c:2415
 kthread+0x357/0x430 kernel/kthread.c:253
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352

Reported-by: syzbot+e863893591cc7a622e40@syzkaller.appspotmail.com
Fixes: c55c8eda ("tipc: smooth change between replicast and broadcast")
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>