io_uring: fail links if msg-ring doesn't succeeed

We must always call req_set_fail() if the request is failed, otherwise
we won't sever links for dependent chains correctly.

Fixes: 4f57f06c

 ("io_uring: add support for IORING_OP_MSG_RING command")
Signed-off-by: Jens Axboe <axboe@kernel.dk>