cgroup: fix spurious warnings on cgroup_is_dead() from cgroup_sk_alloc()

cgroup_get() expected to be called only on live cgroups and triggers
warning on a dead cgroup; however, cgroup_sk_alloc() may be called
while cloning a socket which is left in an empty and removed cgroup
and thus may legitimately duplicate its reference on a dead cgroup.
This currently triggers the following warning spuriously.

 WARNING: CPU: 14 PID: 0 at kernel/cgroup.c:490 cgroup_get+0x55/0x60
 ...
  [<ffffffff8107e123>] __warn+0xd3/0xf0
  [<ffffffff8107e20e>] warn_slowpath_null+0x1e/0x20
  [<ffffffff810ff465>] cgroup_get+0x55/0x60
  [<ffffffff81106061>] cgroup_sk_alloc+0x51/0xe0
  [<ffffffff81761beb>] sk_clone_lock+0x2db/0x390
  [<ffffffff817cce06>] inet_csk_clone_lock+0x16/0xc0
  [<ffffffff817e8173>] tcp_create_openreq_child+0x23/0x4b0
  [<ffffffff818601a1>] tcp_v6_syn_recv_sock+0x91/0x670
  [<ffffffff817e8b16>] tcp_check_req+0x3a6/0x4e0
  [<ffffffff81861ba3>] tcp_v6_rcv+0x693/0xa00
  [<ffffffff81837429>] ip6_input_finish+0x59/0x3e0
  [<ffffffff81837cb2>] ip6_input+0x32/0xb0
  [<ffffffff81837387>] ip6_rcv_finish+0x57/0xa0
  [<ffffffff81837ac8>] ipv6_rcv+0x318/0x4d0
  [<ffffffff817778c7>] __netif_receive_skb_core+0x2d7/0x9a0
  [<ffffffff81777fa6>] __netif_receive_skb+0x16/0x70
  [<ffffffff81778023>] netif_receive_skb_internal+0x23/0x80
  [<ffffffff817787d8>] napi_gro_frags+0x208/0x270
  [<ffffffff8168a9ec>] mlx4_en_process_rx_cq+0x74c/0xf40
  [<ffffffff8168b270>] mlx4_en_poll_rx_cq+0x30/0x90
  [<ffffffff81778b30>] net_rx_action+0x210/0x350
  [<ffffffff8188c426>] __do_softirq+0x106/0x2c7
  [<ffffffff81082bad>] irq_exit+0x9d/0xa0 [<ffffffff8188c0e4>] do_IRQ+0x54/0xd0
  [<ffffffff8188a63f>] common_interrupt+0x7f/0x7f <EOI>
  [<ffffffff8173d7e7>] cpuidle_enter+0x17/0x20
  [<ffffffff810bdfd9>] cpu_startup_entry+0x2a9/0x2f0
  [<ffffffff8103edd1>] start_secondary+0xf1/0x100

This patch renames the existing cgroup_get() with the dead cgroup
warning to cgroup_get_live() after cgroup_kn_lock_live() and
introduces the new cgroup_get() which doesn't check whether the cgroup
is live or dead.

All existing cgroup_get() users except for cgroup_sk_alloc() are
converted to use cgroup_get_live().

Fixes: d979a39d ("cgroup: duplicate cgroup reference when cloning sockets")
Cc: stable@vger.kernel.org # v4.5+
Cc: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Chris Mason <clm@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>