[SK_BUFF]: Use the helpers to get the layer header pointer

Some more cases...

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

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

diff --cc net/ipv6/exthdrs.c
Simple merge

diff --cc net/ipv6/netfilter/nf_conntrack_reasm.c
Simple merge