netfilter: bridge: kill nf_bridge_pad

The br_netfilter frag output function calls skb_cow_head() so in
case it needs a larger headroom to e.g. re-add a previously stripped PPPOE
or VLAN header things will still work (at cost of reallocation).

We can then move nf_bridge_encap_header_len to br_netfilter.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

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

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

diff --cc net/ipv4/ip_output.c
Simple merge