io_uring/futex: get rid of struct io_futex addr union

Rather than use a union of a u32 and struct futex_waitv user address,
consolidate it into a single void __user pointer instead. This also
makes prep easier to use as the store happens to the member that will
be used.

No functional changes in this patch.

Signed-off-by: Jens Axboe <axboe@kernel.dk>