iio: imu: st_lsm6dsx: fix stack leak in tagged FIFO buffer

In the Linux kernel, the following vulnerability has been resolved:

iio: imu: st_lsm6dsx: fix stack leak in tagged FIFO buffer

The tagged FIFO path declares iio_buff on the stack with __aligned(8)

but no initializer, but there is a hole in the structure, which will

then leak to userspace as ST_LSM6DSX_SAMPLE_SIZE bytes (6) will be

copied, but the space between that and the timestamp are not

initialized.

Commit c14edb4d0bdc ("iio:imu:st_lsm6dsx Fix alignment and data leak

issues") moved the untagged FIFO path to a kzalloc'd buffer in hw->scan,

but for the tagged path it only added the alignment qualifier and not

the initializer :(

Fix this by just zero-initializing the structure on the stack.