ulogd2: avoid use after free in unregister on global ulogd_fds linked list

ulogd2: avoid use after free in unregister on global ulogd_fds linked list

Invalid read of size 4
   at 0x405F60: ulogd_unregister_fd (select.c:74)
   by 0x4E4E3DF: ??? (in /usr/lib/ulogd/ulogd_inppkt_NFLOG.so)
   by 0x405003: stop_pluginstances (ulogd.c:1335)
   by 0x405003: sigterm_handler_task (ulogd.c:1383)
   by 0x405153: call_signal_handler_tasks (ulogd.c:424)
   by 0x405153: signal_channel_callback (ulogd.c:443)
   by 0x406163: ulogd_select_main (select.c:105)
   by 0x403CF3: ulogd_main_loop (ulogd.c:1070)
   by 0x403CF3: main (ulogd.c:1649)

Problem is that ulogd_inppkt_NFLOG.c::stop() calls ulogd_unregister_fd()
which does llist_del().  This llist_del may touch ->prev pointer.

As the list element is in private data, we cannot do this llist_del
from stop_pluginstances().

Therefore, the free() process moved location after finishing ulogd_unregister_fd().

Signed-off-by: Kyuwon Shim <kyuwon.shim@alliedtelesis.co.nz>
Signed-off-by: Florian Westphal <fw@strlen.de>