netfilter: bridge: move DNAT helper to br_netfilter

Only one caller, there is no need to keep this in a header.
Move it to br_netfilter.c where this belongs to.

Based on patch from Florian Westphal.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --cc include/linux/netfilter_bridge.h
Simple merge

diff --cc net/bridge/br_device.c
Simple merge

diff --cc net/bridge/br_netfilter.c
Simple merge

diff --cc net/bridge/br_private.h
Simple merge