io_uring: clear opcode specific data for an early failure

If failure happens before the opcode prep handler is called, ensure that
we clear the opcode specific area of the request, which holds data
specific to that request type. This prevents errors where opcode
handlers either don't get to clear per-request private data since prep
isn't even called.

Reported-and-tested-by:  <syzbot+f8e9a371388aa62ecab4@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>