watchdog: Fix the race between the release of watchdog_core_data and cdev

The struct cdev is embedded in the struct watchdog_core_data. In the
current code, we manage the watchdog_core_data with a kref, but the
cdev is manged by a kobject. There is no any relationship between
this kref and kobject. So it is possible that the watchdog_core_data is
freed before the cdev is entirely released. We can easily get the
following call trace with CONFIG_DEBUG_KOBJECT_RELEASE and
CONFIG_DEBUG_OBJECTS_TIMERS enabled.
  ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x38
  WARNING: CPU: 23 PID: 1028 at lib/debugobjects.c:481 debug_print_object+0xb0/0xf0
  Modules linked in: softdog(-) deflate ctr twofish_generic twofish_common camellia_generic serpent_generic blowfish_generic blowfish_common cast5_generic cast_common cmac xcbc af_key sch_fq_codel openvswitch nsh nf_conncount nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4
  CPU: 23 PID: 1028 Comm: modprobe Not tainted 5.3.0-next-20190924-yoctodev-standard+ #180
  Hardware name: Marvell OcteonTX CN96XX board (DT)
  pstate: 00400009 (nzcv daif +PAN -UAO)
  pc : debug_print_object+0xb0/0xf0
  lr : debug_print_object+0xb0/0xf0
  sp : ffff80001cbcfc70
  x29: ffff80001cbcfc70 x28: ffff800010ea2128
  x27: ffff800010bad000 x26: 0000000000000000
  x25: ffff80001103c640 x24: ffff80001107b268
  x23: ffff800010bad9e8 x22: ffff800010ea2128
  x21: ffff000bc2c62af8 x20: ffff80001103c600
  x19: ffff800010e867d8 x18: 0000000000000060
  x17: 0000000000000000 x16: 0000000000000000
  x15: ffff000bd7240470 x14: 6e6968207473696c
  x13: 5f72656d6974203a x12: 6570797420746365
  x11: 6a626f2029302065 x10: 7461747320657669
  x9 : 7463612820657669 x8 : 3378302f3078302b
  x7 : 0000000000001d7a x6 : ffff800010fd5889
  x5 : 0000000000000000 x4 : 0000000000000000
  x3 : 0000000000000000 x2 : ffff000bff948548
  x1 : 276a1c9e1edc2300 x0 : 0000000000000000
  Call trace:
   debug_print_object+0xb0/0xf0
   debug_check_no_obj_freed+0x1e8/0x210
   kfree+0x1b8/0x368
   watchdog_cdev_unregister+0x88/0xc8
   watchdog_dev_unregister+0x38/0x48
   watchdog_unregister_device+0xa8/0x100
   softdog_exit+0x18/0xfec4 [softdog]
   __arm64_sys_delete_module+0x174/0x200
   el0_svc_handler+0xd0/0x1c8
   el0_svc+0x8/0xc

This is a common issue when using cdev embedded in a struct.
Fortunately, we already have a mechanism to solve this kind of issue.
Please see commit 233ed09d ("chardev: add helper function to
register char devs with a struct device") for more detail.

In this patch, we choose to embed the struct device into the
watchdog_core_data, and use the API provided by the commit 233ed09d
to make sure that the release of watchdog_core_data and cdev are
in sequence.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20191008112934.29669-1-haokexin@gmail.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>