netfilter: rcu sparse cleanups

Use RCU helpers to reduce number of sparse warnings
(CONFIG_SPARSE_RCU_POINTER=y), and adds lockdep checks.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

diff --cc net/netfilter/nf_conntrack_expect.c
Simple merge

diff --cc net/netfilter/nf_conntrack_extend.c
Simple merge

diff --cc net/netfilter/nf_conntrack_helper.c
Simple merge

diff --cc net/netfilter/nf_conntrack_proto.c
Simple merge