iio: core: Fix double free.

iio: core: Fix double free.

commit c1b03ab5e886760bdd38c9c7a27af149046ffe01 upstream.

When an error occurred during event registration memory was freed twice
resulting in kernel memory corruption and a crash in unrelated code.

The problem was caused by
	iio_device_unregister_eventset()
	iio_device_unregister_sysfs()

being called twice, once on the error path and then
again via iio_dev_release().

Fix this by making these two functions idempotent so they
may be called multiple times.

The problem was observed before applying
	78b33216 iio:core: Handle error when mask type is not separate

Signed-off-by: Martin Fuzzey <mfuzzey@parkeon.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
[bwh: Backported to 3.2:
 - Adjust filenames, context
 - Drop inapplicable change to iio_free_chan_devattr_list()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>