io_uring: fix zero len do_splice()

do_splice() doesn't expect len to be 0. Just always return 0 in this
case as splice(2) does.

Fixes: 7d67af2c ("io_uring: add splice(2) support")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>