net: initialize init_net earlier

While testing a patch that will follow later
("net: add netns refcount tracker to struct nsproxy")
I found that devtmpfs_init() was called before init_net
was initialized.

This is a bug, because devtmpfs_setup() calls
ksys_unshare(CLONE_NEWNS);

This has the effect of increasing init_net refcount,
which will be later overwritten to 1, as part of setup_net(&init_net)

We had too many prior patches [1] trying to work around the root cause.

Really, make sure init_net is in BSS section, and that net_ns_init()
is called earlier at boot time.

Note that another patch ("vfs: add netns refcount tracker
to struct fs_context") also will need net_ns_init() being called
before vfs_caches_init()

As a bonus, this patch saves around 4KB in .data section.

[1]

f8c46cb3 ("netns: do not call pernet ops for not yet set up init_net namespace")
b5082df8 ("net: Initialise init_net.count to 1")
734b6541 ("net: Statically initialize init_net.dev_base_head")

v2: fixed a build error reported by kernel build bots (CONFIG_NET=n)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>