netfilter: nf_conntrack: use hlist_add_head_rcu() in nf_conntrack_set_hashsize()

Using hlist_add_head() in nf_conntrack_set_hashsize() is quite dangerous.
Without any barrier, one CPU could see a loop while doing its lookup.
Its true new table cannot be seen by another cpu, but previous table is still
readable.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>