V4L/DVB: v4l2: add core serialization lock

Drivers can optionally set a pointer to a mutex in struct video_device.
The core will use that to lock before calling open, read, write, unlocked_ioctl,
poll, mmap or release.

Updated the documentation as well and ensure that v4l2-event knows about the
lock: it will unlock it before doing a blocking wait on an event and relock it
afterwards.

Ensure that the 'video_is_registered' check is done when the lock is held:
a typical disconnect will take the lock as well before unregistering the
device nodes, so to prevent race conditions the video_is_registered check
should also be done with the lock held.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

include/media/v4l2-dev.h

@@ -94,6 +94,9 @@ struct video_device
 
 	/* ioctl callbacks */
 	const struct v4l2_ioctl_ops *ioctl_ops;
+
+	/* serialization lock */
+	struct mutex *lock;
 };
 
 /* dev to video-device */