io_uring/futex: mark wait requests as inflight

Inflight marking is used so that do_exit() -> io_uring_files_cancel()
will find requests with files that reference an io_uring instance,
so they can get appropriately canceled before the files go away.
However, it's also called before the mm goes away.

Mark futex/futexv wait requests as being inflight, so that
io_uring_files_cancel() will prune them. This ensures that the mm stays
alive, which is important as an exiting mm will also free the futex
private hash buckets. An io_uring futex request with FUTEX2_PRIVATE
set relies on those being alive until the request has completed. A
recent commit added these futex private hashes, which get killed when
the mm goes away.

Fixes: 80367ad0 ("futex: Add basic infrastructure for local task local hash")
Link: https://lore.kernel.org/io-uring/38053.1749045482@localhost/


Reported-by: Robert Morris <rtm@csail.mit.edu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>