ipv6: fix cleanup ordering for pingv6 registration

Commit 6d0bfe226116 ("net: ipv6: Add IPv6 support to the ping socket.")
contains an error in the cleanup path of inet6_init(): when
proto_register(&pingv6_prot, 1) fails, we try to unregister
&pingv6_prot. When rawv6_init() fails, we skip unregistering
&pingv6_prot.

Example of panic (triggered by faking a failure of
 proto_register(&pingv6_prot, 1)):

    general protection fault: 0000 [#1] PREEMPT SMP KASAN PTI
    [...]
    RIP: 0010:__list_del_entry_valid+0x79/0x160
    [...]
    Call Trace:
     proto_unregister+0xbb/0x550
     ? trace_preempt_on+0x6f0/0x6f0
     ? sock_no_shutdown+0x10/0x10
     inet6_init+0x153/0x1b8

Fixes: 6d0bfe226116 ("net: ipv6: Add IPv6 support to the ping socket.")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/af_inet6.c

@@ -938,14 +938,14 @@ static int __init inet6_init(void)
 
 	err = proto_register(&pingv6_prot, 1);
 	if (err)
-		goto out_unregister_ping_proto;
+		goto out_unregister_raw_proto;
 
 	/* We MUST register RAW sockets before we create the ICMP6,
 	 * IGMP6, or NDISC control sockets.
 	 */
 	err = rawv6_init();
 	if (err)
-		goto out_unregister_raw_proto;
+		goto out_unregister_ping_proto;
 
 	/* Register the family here so that the init calls below will
 	 * be able to create sockets. (?? is this dangerous ??)