netfilter: nf_nat: fix RCU races

Fix three ct_extend/NAT extension related races:

- When cleaning up the extension area and removing it from the bysource hash,
  the nat->ct pointer must not be set to NULL since it may still be used in
  a RCU read side

- When replacing a NAT extension area in the bysource hash, the nat->ct
  pointer must be assigned before performing the replacement

- When reallocating extension storage in ct_extend, the old memory must
  not be freed immediately since it may still be used by a RCU read side

Possibly fixes https://bugzilla.redhat.com/show_bug.cgi?id=449315
and/or http://bugzilla.kernel.org/show_bug.cgi?id=10875

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>