Fix batch forget can't handle too large msg

[zyfjeff]

error: cannot process fuse message: Value too large for defined data type

By default, when fuse sever reads the batch forget request, the bufsize given is MAX_BUFFER_SIZE + BUFFER_HEADER_SIZE, and the fuse kernel will fill this buffer as much as possible, so the size of the data that batch forget can fill should be the total buffer size - fuse header - batch forget header

Here's the code for the fuse kernel to fill the batch forget request.

    struct fuse_batch_forget_in arg = { .count = 0 };
    struct fuse_in_header ih = {
        .opcode = FUSE_BATCH_FORGET,
        .unique = fuse_get_unique(fiq),
        .len = sizeof(ih) + sizeof(arg),
    };

    if (nbytes < ih.len) {
        spin_unlock(&fiq->lock);
        return -EINVAL;
    }

    max_forgets = (nbytes - ih.len) / sizeof(struct fuse_forget_one);
    head = fuse_dequeue_forget(fiq, max_forgets, &count);
    spin_unlock(&fiq->lock);

    arg.count = count;
    ih.len += count * sizeof(struct fuse_forget_one);

so fuse_batch_forget_in.count * sizeof(struct fuse_forget_one) should less than or equal to bufsize - sizeof(fuse_in_header) - sizeof(fuse_batch_forget_in)