net: Order blackhole_netdev_init(), inet_init(), and inet6_init().

Commit 309b905dee ("ipv6: convert CONFIG_IPV6 to built-in
only and clean up Kconfigs") started to call inet6_init() as
device_initcall().

It explains the reason as a race condition on IPv6 addrconf,
but this is not accurate.

The change initially used fs_initcall() but switched to
device_initcall() due to the null-ptr-deref stack trace in
the link below.

It happened because blackhole_netdev_init() uses device_initcall()
and blackhole_netdev was NULL when addrconf_init() called
ipv6_add_dev(blackhole_netdev).

Due to the order in Kbuild,

  $ cat -n Kbuild | grep -E "(drivers|net)"
     111	obj-y			+= drivers/
     114	obj-$(CONFIG_NET)	+= net/

using device_initcall() for IPv6 allows built-in drivers to
be initialised earlier, which requires unnecessary NULL checks.

For the same ordering reason, IPv4 uses late_initcall() to
initialise blackhole_netdev.

Given loopback_net_ops is registered in net_dev_init() at
subsys_initcall(), blackhole_netdev_init() can be called
after that.

In addition, some places assume that IPv4 must be initialised
before IPv6.  For example, mptcp_proto_v6_init() copies
mptcp_prot to mptcp_v6_prot, which would otherwise have NULL
mptcp_v6_prot.h.hashinfo.

Let's explicitly order blackhole_netdev_init() -> inet_init()
-> inet6_init() before device_initcall() with 3 different
initcall levels:

  subsys_initcall      : 4  : net_dev_init()
  subsys_initcall_sync : 4s : blackhole_netdev_init()
  fs_initcall          : 5  : inet_init()
  fs_initcall_sync     : 5s : inet6_init()
  device_initcall      : 6  : built-in drivers

Note that both IPv4 and IPv6 can still use fs_initcall() thanks
to the order in net/Makefile, but explicit ordering would be
less error-prone.

  $ cat -n net/Makefile | grep ipv
      17	obj-$(CONFIG_INET)		+= ipv4/
      22	obj-y				+= ipv6/

Link: https://lore.kernel.org/netdev/20260309074758.0ea95a18@kernel.org/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Chris J Arges <carges@cloudflare.com>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20261001204752.2572265-2-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/loopback.c

@@ -289,4 +289,4 @@ static int __init blackhole_netdev_init(void)
 	return 0;
 }
 
-device_initcall(blackhole_netdev_init);
+subsys_initcall_sync(blackhole_netdev_init);

net/ipv6/af_inet6.c

@@ -1217,4 +1217,4 @@ static int __init inet6_init(void)
 	proto_unregister(&tcpv6_prot);
 	goto out;
 }
-device_initcall(inet6_init);
+fs_initcall_sync(inet6_init);