[media] dvb_frontend: eliminate blocking wait in dvb_unregister_frontend()

[media] dvb_frontend: eliminate blocking wait in dvb_unregister_frontend()

The wait_event() call in dvb_unregister_frontend() waits synchronously
for other tasks to free a file descriptor, but it does that while
holding several mutexes.  That alone is a bad idea, but if one user
process happens to keep a (defunct) file descriptor open indefinitely,
the kernel will correctly detect a hung task:

    INFO: task kworker/0:1:314 blocked for more than 30 seconds.
          Not tainted 4.7.0-rc1-hosting+ #50
    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    kworker/0:1     D ffff88003daf7a50     0   314      2 0x00000000
    Workqueue: usb_hub_wq hub_event
     ffff88003daf7a50 0000000000000296 ffff88003daf7a30 ffff88003fc13f98
     ffff88003dadce00 ffff88003daf8000 ffff88003e3fc010 ffff88003d48d4f8
     ffff88003e3b5030 ffff88003e3f8898 ffff88003daf7a68 ffffffff810cf860
    Call Trace:
     [<ffffffff810cf860>] schedule+0x30/0x80
     [<ffffffff812f88d3>] dvb_unregister_frontend+0x93/0xc0
     [<ffffffff8107a000>] ? __wake_up_common+0x80/0x80
     [<ffffffff813019c7>] dvb_usb_adapter_frontend_exit+0x37/0x70
     [<ffffffff81300614>] dvb_usb_exit+0x34/0xb0
     [<ffffffff81300d4a>] dvb_usb_device_exit+0x3a/0x50
     [<ffffffff81302dc2>] pctv452e_usb_disconnect+0x52/0x60
     [<ffffffff81295a07>] usb_unbind_interface+0x67/0x1e0
     [<ffffffff810609f3>] ? __blocking_notifier_call_chain+0x53/0x70
     [<ffffffff8127ba67>] __device_release_driver+0x77/0x110
     [<ffffffff8127c2d3>] device_release_driver+0x23/0x30
     [<ffffffff8127ab5d>] bus_remove_device+0x10d/0x150
     [<ffffffff8127879b>] device_del+0x13b/0x260
     [<ffffffff81299dea>] ? usb_remove_ep_devs+0x1a/0x30
     [<ffffffff8129468e>] usb_disable_device+0x9e/0x1e0
     [<ffffffff8128bb09>] usb_disconnect+0x89/0x260
     [<ffffffff8128db8d>] hub_event+0x30d/0xfc0
     [<ffffffff81059475>] process_one_work+0x1c5/0x4a0
     [<ffffffff8105940c>] ? process_one_work+0x15c/0x4a0
     [<ffffffff81059799>] worker_thread+0x49/0x480
     [<ffffffff81059750>] ? process_one_work+0x4a0/0x4a0
     [<ffffffff81059750>] ? process_one_work+0x4a0/0x4a0
     [<ffffffff8105f65e>] kthread+0xee/0x110
     [<ffffffff810400bf>] ret_from_fork+0x1f/0x40
     [<ffffffff8105f570>] ? __kthread_unpark+0x70/0x70
    5 locks held by kworker/0:1/314:
     #0:  ("usb_hub_wq"){......}, at: [<ffffffff8105940c>] process_one_work+0x15c/0x4a0
     #1:  ((&hub->events)){......}, at: [<ffffffff8105940c>] process_one_work+0x15c/0x4a0
     #2:  (&dev->mutex){......}, at: [<ffffffff8128d8cb>] hub_event+0x4b/0xfc0
     #3:  (&dev->mutex){......}, at: [<ffffffff8128bad2>] usb_disconnect+0x52/0x260
     #4:  (&dev->mutex){......}, at: [<ffffffff8127c2cb>] device_release_driver+0x1b/0x30

This patch removes the blocking wait, and postpones the kfree() call
until all file handles have been closed by using struct kref.

Signed-off-by: Max Kellermann <max@duempel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>