netfilter: nat: fix crash when conntrack entry is re-used
Stas Nichiporovich reports oops in nf_nat_bysource_cmp(), trying to
access nf_conn struct at address 0xffffffffffffff50.
This is the result of fetching a null rhash list (struct embedded at
offset 176; 0 - 176 gets us ...fff50).
The problem is that conntrack entries are allocated from a
SLAB_DESTROY_BY_RCU cache, i.e. entries can be free'd and reused
on another cpu while nf nat bysource hash access the same conntrack entry.
Freeing is fine (we hold rcu read lock); zeroing rhlist_head isn't.
-> Move the rhlist struct outside of the memset()-inited area.