ALSA: usb-audio: Clamp implicit feedback packet count to URB capacity

In the Linux kernel, the following vulnerability has been resolved:

ALSA: usb-audio: Clamp implicit feedback packet count to URB capacity

data_ep_set_params() allocates each data URB for exactly u->packets
isochronous frames, so urb->iso_frame_desc[] has u->packets slots and
ctx->packets is the driver's only record of that limit. For an implicit
feedback sink, snd_usb_queue_pending_output_urbs() overwrites it with the
sync source's packet count, which is calculated independently from the
capture endpoint's parameters. When that count is larger,
prepare_playback_urb() and prepare_silent_urb() can write
iso_frame_desc[] past the allocation; their existing bounds limit payload
bytes, not the descriptor index.

The reproducer uses a high-speed UAC2 device declaring bInterval 1 for
implicit feedback capture (8 packets) and bInterval 4 for playback
(1 packet). On the first capture completion after the stream starts, it
accesses seven descriptors spanning 112 bytes beyond the one-packet URB:

BUG: KASAN: slab-out-of-bounds in prepare_playback_urb (sound/usb/pcm.c:1560)
Write of size 4 at addr ffff88801e696ad0 by task vhci_rx/178
prepare_playback_urb (sound/usb/pcm.c:1560)
prepare_outbound_urb (sound/usb/endpoint.c:340)
snd_usb_queue_pending_output_urbs (sound/usb/endpoint.c:501)
snd_complete_urb (sound/usb/endpoint.c:1834)
__usb_hcd_giveback_urb (drivers/usb/core/hcd.c:1657)
usb_hcd_giveback_urb (drivers/usb/core/hcd.c:1741)
vhci_rx_loop (drivers/usb/usbip/vhci_rx.c:107)
kthread (kernel/kthread.c:436)
The buggy address belongs to the object at ffff88801e696a00
which belongs to the cache kmalloc-256 of size 256
The buggy address is located 0 bytes to the right of
allocated 208-byte region [ffff88801e696a00, ffff88801e696ad0)

Record the allocated packet count per endpoint and clamp both the adopted
count and the packet-size copy to it. Fold the Format Type II delimiter
into urb_packs before the allocation loop so the recorded limit matches
every URB.