[INET_DIAG]: Fix inet_diag_lock_handler error path.

Fixes: http://bugzilla.kernel.org/show_bug.cgi?id=9825

The inet_diag_lock_handler function uses ERR_PTR to encode errors but
its callers were testing against NULL.

This only happens when the only inet_diag modular user, DCCP, is not
built into the kernel or available as a module.

Also there was a problem with not dropping the mutex lock when a handler
was not found, also fixed in this patch.

This caused an OOPS and ss would then hang on subsequent calls, as
&inet_diag_table_mutex was being left locked.

Thanks to spike at ml.yaroslavl.ru for report it after trying 'ss -d'
on a kernel that doesn't have DCCP available.

This bug was introduced in cset
d523a328 ("Fix inet_diag dead-lock
regression"), after 2.6.24-rc3, so just 2.6.24 seems to be affected.
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/inet_diag.c

@@ -259,8 +259,10 @@ static int inet_diag_get_exact(struct sk_buff *in_skb,
 	const struct inet_diag_handler *handler;
 
 	handler = inet_diag_lock_handler(nlh->nlmsg_type);
-	if (!handler)
-		return -ENOENT;
+	if (IS_ERR(handler)) {
+		err = PTR_ERR(handler);
+		goto unlock;
+	}
 
 	hashinfo = handler->idiag_hashinfo;
 	err = -EINVAL;
@@ -708,8 +710,8 @@ static int inet_diag_dump(struct sk_buff *skb, struct netlink_callback *cb)
 	struct inet_hashinfo *hashinfo;
 
 	handler = inet_diag_lock_handler(cb->nlh->nlmsg_type);
-	if (!handler)
-		goto no_handler;
+	if (IS_ERR(handler))
+		goto unlock;
 
 	hashinfo = handler->idiag_hashinfo;
 
@@ -838,7 +840,6 @@ static int inet_diag_dump(struct sk_buff *skb, struct netlink_callback *cb)
 	cb->args[2] = num;
 unlock:
 	inet_diag_unlock_handler(handler);
-no_handler:
 	return skb->len;
 }