The following traceback is sometimes seen when booting an image in qemu:

[   54.608293] cdrom: Uniform CD-ROM driver Revision: 3.20
[   54.611085] Fusion MPT base driver 3.04.20
[   54.611877] Copyright (c) 1999-2008 LSI Corporation
[   54.616234] Fusion MPT SAS Host driver 3.04.20
[   54.635139] sysctl duplicate entry: /dev/cdrom//info
[   54.639578] CPU: 0 PID: 266 Comm: kworker/u4:5 Not tainted 5.0.0-rc5 #1
[   54.639578] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
[   54.641273] Workqueue: events_unbound async_run_entry_fn
[   54.641273] Call Trace:
[   54.641273]  dump_stack+0x67/0x90
[   54.641273]  __register_sysctl_table+0x50b/0x570
[   54.641273]  ? rcu_read_lock_sched_held+0x6f/0x80
[   54.641273]  ? kmem_cache_alloc_trace+0x1c7/0x1f0
[   54.646814]  __register_sysctl_paths+0x1c8/0x1f0
[   54.646814]  cdrom_sysctl_register.part.7+0xc/0x5f
[   54.646814]  register_cdrom.cold.24+0x2a/0x33
[   54.646814]  sr_probe+0x4bd/0x580
[   54.646814]  ? __driver_attach+0xd0/0xd0
[   54.646814]  really_probe+0xd6/0x260
[   54.646814]  ? __driver_attach+0xd0/0xd0
[   54.646814]  driver_probe_device+0x4a/0xb0
[   54.646814]  ? __driver_attach+0xd0/0xd0
[   54.646814]  bus_for_each_drv+0x73/0xc0
[   54.646814]  __device_attach+0xd6/0x130
[   54.646814]  bus_probe_device+0x9a/0xb0
[   54.646814]  device_add+0x40c/0x670
[   54.646814]  ? __pm_runtime_resume+0x4f/0x80
[   54.646814]  scsi_sysfs_add_sdev+0x81/0x290
[   54.646814]  scsi_probe_and_add_lun+0x888/0xc00
[   54.646814]  ? scsi_autopm_get_host+0x21/0x40
[   54.646814]  __scsi_add_device+0x116/0x130
[   54.646814]  ata_scsi_scan_host+0x93/0x1c0
[   54.646814]  async_run_entry_fn+0x34/0x100
[   54.646814]  process_one_work+0x237/0x5e0
[   54.646814]  worker_thread+0x37/0x380
[   54.646814]  ? rescuer_thread+0x360/0x360
[   54.646814]  kthread+0x118/0x130
[   54.646814]  ? kthread_create_on_node+0x60/0x60
[   54.646814]  ret_from_fork+0x3a/0x50

The only sensible explanation is that cdrom_sysctl_register() is called
twice, once from the module init function and once from register_cdrom().
cdrom_sysctl_register() is not mutex protected and may happily execute
twice if the second call is made before the first call is complete.

Use a static atomic to ensure that the function is executed exactly once.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jens Axboe <axboe@kernel.dk>