iio: buffer: use sysfs_attr_init() on allocated attrs

iio: buffer: use sysfs_attr_init() on allocated attrs

When dynamically allocating sysfs attributes, it's a good idea to call
sysfs_attr_init() on them to initialize lock_class_keys.
This change does that.

The lock_class_keys are set when the CONFIG_DEBUG_LOCK_ALLOC symbol is
enabled. Which is [likely] one reason why I did not see this during
development.

I also am not able to see this even with CONFIG_DEBUG_LOCK_ALLOC enabled,
so this may [likely] be reproduce-able on some system configurations.

This was reported via:
  https://lore.kernel.org/linux-iio/CA+U=DsrsvGgXEF30-vXuXS_k=-mjSjiBwEEzwKb1hJVn1P98OA@mail.gmail.com/T/#u

Fixes: 15097c7a1adc ("iio: buffer: wrap all buffer attributes into iio_dev_attr")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210402174226.630346-1-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>