netfilter: nf_log regression fix

commit ca735b3a
'netfilter: use a linked list of loggers'
introduced an array of list_head in "struct nf_logger", but
forgot to initialize it in nf_log_register(). This resulted
in oops when calling nf_log_unregister() at module unload time.

Reported-and-tested-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Eric Leblond <eric@inl.fr>
Signed-off-by: Patrick McHardy <kaber@trash.net>