[NETFILTER]: {eb,ip6,ip}t_LOG: remove remains of LOG target overloading

All LOG targets always use their internal logging function nowadays, so
remove the incorrect error message and handle real errors (!= -EEXIST)
by failing to load.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>