[ALSA] Handle file operations during snd_card disconnects using static file->f_op

Alsa used to kmalloc one file->f_op per file per disconnecting snd_card.
This led to oopses sometimes when file->f_op was freed before __fput()
finished.
Patch adds a virtual device for disconnect: VDD.
VDD consists of:
	LIST_HEAD(shutdown_files)
	    protected by DEFINE_SPINLOCK(shutdown_mutex)
	static struct file_operations snd_shutdown_f_ops
	    and functions assigned to it
	Additions to struct snd_monitor_file
	    to specify if instance is hidden by VDD or not.
A VDD's instance is
	created in snd_card_disconnect() under the card->files_lock.
	cleaned up in snd_card_file_remove() under the card->files_lock.

Signed-off-by: Karsten Wiese <fzu@wemgehoertderstaat.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>