em_ipset: Build on old kernels

em_ipset: Build on old kernels

Make sure TCF_EM_IPSET defined and corresponds to current upstream value
if not defined in target kernel. You need iproute2 version that supports
em_ipset to communicate correctly. Include ip_set_compat.h after
pkt_cls.h to prevent TCF_EM_IPSET redefine error.

Detect skb->iif => skb->skb_iif rename after commit 8964be4a9a5c ("net:
rename skb->iif to skb->skb_iif").

Add dev_get_by_index_rcu() define pointing to __dev_get_by_index() to
build on RHEL6 kernels with explicit note that this may not work on all
architectures.

Always build em_ipset regardless of CONFIG_NET_EMATCH_IPSET option.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>