ipv4: initialize fib_trie prior to register_netdev_notifier call.

Net stack initialization currently initializes fib-trie after the
first call to netdevice_notifier() call. In fact fib_trie initialization
needs to happen before first rtnl_register(). It does not cause any problem
since there are no devices UP at this moment, but trying to bring 'lo'
UP at initialization would make this assumption wrong and exposes the issue.

Fixes following crash

 Call Trace:
  ? alternate_node_alloc+0x76/0xa0
  fib_table_insert+0x1b7/0x4b0
  fib_magic.isra.17+0xea/0x120
  fib_add_ifaddr+0x7b/0x190
  fib_netdev_event+0xc0/0x130
  register_netdevice_notifier+0x1c1/0x1d0
  ip_fib_init+0x72/0x85
  ip_rt_init+0x187/0x1e9
  ip_init+0xe/0x1a
  inet_init+0x171/0x26c
  ? ipv4_offload_init+0x66/0x66
  do_one_initcall+0x43/0x160
  kernel_init_freeable+0x191/0x219
  ? rest_init+0x80/0x80
  kernel_init+0xe/0x150
  ret_from_fork+0x22/0x30
 Code: f6 46 23 04 74 86 4c 89 f7 e8 ae 45 01 00 49 89 c7 4d 85 ff 0f 85 7b ff ff ff 31 db eb 08 4c 89 ff e8 16 47 01 00 48 8b 44 24 38 <45> 8b 6e 14 4d 63 76 74 48 89 04 24 0f 1f 44 00 00 48 83 c4 08
 RIP: kmem_cache_alloc+0xcf/0x1c0 RSP: ffff9b1500017c28
 CR2: 0000000000000014

Fixes: 7b1a74fd ("[NETNS]: Refactor fib initialization so it can handle multiple namespaces.")
Fixes: 7f9b8052 ("[IPV4]: fib hash|trie initialization")

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>