kobject: fix NULL pointer derefernce in kobj_child_ns_ops

We will hit NULL pointer dereference if we call
platform_device_register_simple or platform_device_add at very early
stage. I have observed following crash when called platform_device_add
from "init_irq" hook of machine_desc. This patch fixes this issue and
let system handle this case gracefully instead of kernel panic.

[0.000000] Unable to handle kernel NULL pointer dereference at
virtual address 0000000c
[0.000000] pgd = c0004000
[0.000000] [0000000c] *pgd=00000000
[0.000000] Internal error: Oops: 5 [#1] PREEMPT ARM
[0.000000] Modules linked in:
[0.000000] CPU: 0 PID: 0 Comm: swapper Tainted: G        W 3.17.0-rc6-00198-ga1603f1-dirty #319
[0.000000] task: c05b23f0 ti: c05a8000 task.ti: c05a8000
[0.000000] PC is at kobject_namespace+0x18/0x58
[0.000000] LR is at kobject_add_internal+0x90/0x2ec
[snip]
[0.000000] [<c01b1df0>] (kobject_namespace) from [<c01b2338>] (kobject_add_internal+0x90/0x2ec)
[0.000000] [<c01b2338>] (kobject_add_internal) from [<c01b2728>] (kobject_add+0x4c/0x98)
[0.000000] [<c01b2728>] (kobject_add) from [<c0226274>] (device_add+0xe8/0x51c)
[0.000000] [<c0226274>] (device_add) from [<c0229c70>] (platform_device_add+0xb4/0x214)
[0.000000] [<c0229c70>] (platform_device_add) from [<c022a338>] (platform_device_register_full+0xb8/0xdc)
[0.000000] [<c022a338>] (platform_device_register_full) from [<c0570214>] (exynos_init_irq+0x90/0x9c)
[0.000000] [<c0570214>] (exynos_init_irq) from [<c056c18c>] (init_IRQ+0x2c/0x78)
[0.000000] [<c056c18c>] (init_IRQ) from [<c0569a54>] (start_kernel+0x22c/0x378)
[0.000000] [<c0569a54>] (start_kernel) from [<40008070>] (0x40008070)
[0.000000] Code: e590000c e3500000 0a00000e e5903014 (e593300c)

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>