wifi: ath9k: hif_usb: fix memory leak of remain_skbs

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

wifi: ath9k: hif_usb: fix memory leak of remain_skbs

hif_dev->remain_skb is allocated and used exclusively in

ath9k_hif_usb_rx_stream(). It is implied that an allocated remain_skb is

processed and subsequently freed (in error paths) only during the next

call of ath9k_hif_usb_rx_stream().

So, if the urbs are deallocated between those two calls due to the device

deinitialization or suspend, it is possible that ath9k_hif_usb_rx_stream()

is not called next time and the allocated remain_skb is leaked. Our local

Syzkaller instance was able to trigger that.

remain_skb makes sense when receiving two consecutive urbs which are

logically linked together, i.e. a specific data field from the first skb

indicates a cached skb to be allocated, memcpy'd with some data and

subsequently processed in the next call to ath9k_hif_usb_rx_stream(). Urbs

deallocation supposedly makes that link irrelevant so we need to free the

cached skb in those cases.

Fix the leak by introducing a function to explicitly free remain_skb (if

it is not NULL) when the rx urbs have been deallocated. remain_skb is NULL

when it has not been allocated at all (hif_dev struct is kzalloced) or

when it has been processed in next call to ath9k_hif_usb_rx_stream().

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.