netfilter: nf_nat_sip: fix mangling

In (b20ab9cc netfilter: nf_ct_helper: better logging for dropped packets)
there were some missing brackets around the logging information, thus
always returning drop.

Closes https://bugzilla.kernel.org/show_bug.cgi?id=60061



Signed-off-by: Balazs Peter Odor <balazs@obiserver.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>