iio: event: Fix event FIFO reset race

Summary

In the Linux kernel, the following vulnerability has been resolved: iio: event: Fix event FIFO reset race `iio_event_getfd()` creates the event file descriptor with `anon_inode_getfd()`, which allocates a new fd, creates the anonymous file and installs it in the process fd table before returning to the caller. The IIO code resets the event FIFO after `anon_inode_getfd()` has returned, but before `IIO_GET_EVENT_FD_IOCTL` has copied the fd number to userspace. But since fd tables are shared between threads, another thread can guess the newly allocated fd number and issue a `read()` on it as soon as the fd has been installed. This means the `kfifo_to_user()` in `iio_event_chrdev_read()` can run in parallel with the `kfifo_reset_out()` in `iio_event_getfd()`. The kfifo documentation says that `kfifo_reset_out()` is only safe when it is called from the reader thread and there is only one concurrent reader. Otherwise it is dangerous and must be handled in the same way as `kfifo_reset()`. If that happens, `kfifo_to_user()` can advance the FIFO `out` index based on state from before the reset, after the reset has already moved the `out` index to the current `in` index. That can leave the FIFO with an `out` index past the `in` index. A later `read()` can then see an underflowed FIFO length and copy more data than the event FIFO buffer contains. This can result in an out-of-bounds read and leak adjacent kernel memory to userspace. Move the FIFO reset before `anon_inode_getfd()`. At that point the event fd is marked busy, but the new fd has not been installed yet, so userspace cannot access it while the FIFO is reset.