sunrpc: fix cache_request leak in cache_release

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

sunrpc: fix cache_request leak in cache_release

When a reader's file descriptor is closed while in the middle of reading

a cache_request (rp->offset != 0), cache_release() decrements the

request's readers count but never checks whether it should free the

request.

In cache_read(), when readers drops to 0 and CACHE_PENDING is clear, the

cache_request is removed from the queue and freed along with its buffer

and cache_head reference. cache_release() lacks this cleanup.

The only other path that frees requests with readers == 0 is

cache_dequeue(), but it runs only when CACHE_PENDING transitions from

set to clear. If that transition already happened while readers was

still non-zero, cache_dequeue() will have skipped the request, and no

subsequent call will clean it up.

Add the same cleanup logic from cache_read() to cache_release(): after

decrementing readers, check if it reached 0 with CACHE_PENDING clear,

and if so, dequeue and free the cache_request.