io_uring: fix incorrect unlikely() usage in io_waitid_prep()

The negation operator is incorrectly placed outside the unlikely()
macro:

    if (!unlikely(iwa))

This inverts the compiler branch prediction hint, marking the NULL case
as likely instead of unlikely. The intent is to indicate that allocation
failures are rare, consistent with common kernel patterns.

 Moving the negation inside unlikely():

    if (unlikely(!iwa))

Fixes: 2b4fc4cd ("io_uring/waitid: setup async data in the prep handler")
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>