tipc: protect node reset trace dump with node lock

In the Linux kernel, the following vulnerability has been resolved:

tipc: protect node reset trace dump with node lock

The tipc_node_reset_links trace event asks tipc_node_dump() to walk the
node's link entries. Unlike the other node events that request link data,
this event runs without the node lock.

This permits bearer teardown to free a link while the trace callback is
dumping it:

CPU 0 CPU 1
trace_tipc_node_reset_links()
tipc_node_dump()
l = n->links[0].link
tipc_node_write_lock()
kfree(l)
n->links[0].link = NULL
tipc_node_write_unlock()
tipc_link_dump(l)

tipc_link_dump() then dereferences the stale pointer. KASAN reported:

BUG: KASAN: slab-use-after-free in tipc_link_dump
Read of size 4 by task poc/115
Call Trace:
tipc_link_dump+0x10cb/0x16b0
tipc_node_dump+0x4bb/0x740
trace_event_raw_event_tipc_node_class+0x258/0x360
tipc_node_reset_links+0x14d/0x1a0
tipc_rcv+0x13f5/0x3030
tipc_udp_recv+0x4e3/0x670
Allocated by task 0:
tipc_link_create+0x1e1/0x1020
tipc_node_check_dest+0x7d2/0x11a0
tipc_disc_rcv+0xdbf/0x1430
Freed by task 89:
kfree+0x131/0x3c0
tipc_node_link_down+0x267/0x4b0
tipc_node_delete_links+0xec/0x160
bearer_disable+0x107/0x260

Take the node write lock around the trace event. This serializes the
dump against tipc_node_link_down(delete=true), which frees the link
under the same write lock.