[NETFILTER 2.4]: Fix deadlock on NAT helper unload

[Patrick McHardy <kaber@trash.net>]

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[NETFILTER]: Fix deadlock on NAT helper unload

When a NAT helper is unlocked conntrack/NAT may deadlock because of
the following lock sequence:

.. ip_nat_helper_unregister
-> ip_ct_selective_cleanup
-> get_next_corpse		(ip_conntrack_lock)
-> kill_helper			(ip_nat_lock)

.. ip_nat_fn			(ip_nat_lock)
-> ip_nat_setup_info
-> ip_conntrack_alter_reply	(ip_conntrack_lock)

Taking ip_nat_lock in kill_helper() is unnecessary since the helper assigned
to a connection is immutable and new connections can't have the helper that
is beeing unloaded assigned since it is already removed from the global list.

Reported by <doublefacer007@gmail.com>.

Signed-off-by: Patrick McHardy <kaber@trash.net>

--- a/net/ipv4/netfilter/ip_nat_helper.c	2006-09-03 16:41:53.000000000 +0200
+++ b/net/ipv4/netfilter/ip_nat_helper.c	2006-09-03 16:42:04.000000000 +0200
@@ -522,13 +522,7 @@
 static int
 kill_helper(const struct ip_conntrack *i, void *helper)
 {
-	int ret;
-
-	READ_LOCK(&ip_nat_lock);
-	ret = (i->nat.info.helper == helper);
-	READ_UNLOCK(&ip_nat_lock);
-
-	return ret;
+	return (i->nat.info.helper == helper);
 }
 
 void ip_nat_helper_unregister(struct ip_nat_helper *me)