io_uring/kbuf: reject zero sized provided buffers

This isn't fixing a real issue, but there's also zero point in going
through group and buffer setup, when the buffers are going to be
rejected once attempted to get used.

Cc: stable@vger.kernel.org
Reported-by: syzbot+58928048fd1416f1457c@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/kbuf.c

@@ -504,6 +504,8 @@ int io_provide_buffers_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe
 	p->nbufs = tmp;
 	p->addr = READ_ONCE(sqe->addr);
 	p->len = READ_ONCE(sqe->len);
+	if (!p->len)
+		return -EINVAL;
 
 	if (check_mul_overflow((unsigned long)p->len, (unsigned long)p->nbufs,
 				&size))