tipc: Fix missing list initializations in struct tipc_subscription

When an item of struct tipc_subscription is created, we fail to
initialize the two lists aggregated into the struct. This has so far
never been a problem, since the items are just added to a root
object by list_add(), which does not require the addee list to be
pre-initialized. However, syzbot is provoking situations where this
addition fails, whereupon the attempted removal if the item from
the list causes a crash.

This problem seems to always have been around, despite that the code
for creating this object was rewritten in commit 242e82cc ("tipc:
collapse subscription creation functions"), which is still in net-next.

We fix this for that commit by initializing the two lists properly.

Fixes: 242e82cc ("tipc: collapse subscription creation functions")
Reported-by:  <syzbot+0bb443b74ce09197e970@syzkaller.appspotmail.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>