[NETFILTER] Fix conntrack event cache deadlock/oops

This patch fixes a number of bugs.  It cannot be reasonably split up in
multiple fixes, since all bugs interact with each other and affect the same
function:

Bug #1:
The event cache code cannot be called while a lock is held.  Therefore, the
call to ip_conntrack_event_cache() within ip_ct_refresh_acct() needs to be
moved outside of the locked section.  This fixes a number of 2.6.14-rcX
oops and deadlock reports.

Bug #2:
We used to call ct_add_counters() for unconfirmed connections without
holding a lock.  Since the add operations are not atomic, we could race
with another CPU.

Bug #3:
ip_ct_refresh_acct() lost REFRESH events in some cases where refresh
(and the corresponding event) are desired, but no accounting shall be
performed.  Both, evenst and accounting implicitly depended on the skb
parameter bein non-null.   We now re-introduce a non-accounting
"ip_ct_refresh()" variant to explicitly state the desired behaviour.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>