fuse-uring: end fuse_req on io-uring cancel task work

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

fuse-uring: end fuse_req on io-uring cancel task work

When io_uring delivers task work with tw.cancel set (PF_EXITING,
PF_KTHREAD fallback, or percpu_ref_is_dying on the ring context),
fuse_uring_send_in_task() takes the cancel branch, assigns
-ECANCELED, and falls through to fuse_uring_send(). That path only
flips the entry to FRRS_USERSPACE and completes the io_uring cmd;
it never discharges the ring entry's owning reference to the
fuse_req that fuse_uring_add_req_to_ring_ent() handed it at
dispatch time.

fuse_uring_send_in_task()
tw.cancel == true
err = -ECANCELED
fuse_uring_send(ent, cmd, err, issue_flags)
ent->state = FRRS_USERSPACE
list_move(&ent->list, &queue->ent_in_userspace)
ent->cmd = NULL
io_uring_cmd_done(-ECANCELED)
/* ent->fuse_req still set, req still hashed */

The fuse_req stays linked on fpq->processing[hash] and
fuse_request_end() is never invoked. The originating syscall
thread blocks in D-state in request_wait_answer() until
fuse_abort_conn() runs, which can be the entire connection
lifetime. For FR_BACKGROUND requests fc->num_background is never
decremented either, so repeated cancels inflate the counter until
max_background is hit and all later background ops stall. tw.cancel does
not imply a connection abort (e.g. a single io_uring worker thread exits
while the fuse connection stays up), so this cannot be left for
fuse_abort_conn() to clean up.

Ending the req but still routing the entry through fuse_uring_send()
is not enough: that leaves a req-less entry on ent_in_userspace, and
ent_list_request_expired() dereferences ent->fuse_req unconditionally
on the head of that list, which would then NULL-deref.

Fix the cancel branch to release the entry directly. Remove it from the
queue, complete the io_uring cmd, end the fuse_req, free the entry, and
drop its queue_refs (waking the teardown waiter if it was the last).