tipc: fix bug on error path in tipc_topsrv_kern_subscr()

In commit cc1ea9ffadf7 ("tipc: eliminate struct tipc_subscriber") we
re-introduced an old bug on the error path in the function
tipc_topsrv_kern_subscr(). We now re-introduce the correction too.

Reported-by:  <syzbot+f62e0f2a0ef578703946@syzkaller.appspotmail.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>