[NETFILTER]: fix format specifier for netfilter log targets

The prefix argument for nf_log_packet is a format specifier,
so don't pass the user defined string directly to it.

Signed-off-by: Philip Craig <philipc@snapgear.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>