iio: buffer: hw-consumer: free scan_mask on buffer release

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

iio: buffer: hw-consumer: free scan_mask on buffer release

The scan_mask lifetime changed in commit 9a2e1233d38c ("iio: buffer:
hw-consumer: remove redundant scan_mask flexible array").

Before that change, the scan mask storage was embedded in struct
hw_consumer_buffer, so iio_hw_buf_release() could free the whole
allocation with a single kfree(hw_buf).

That commit moved the scan mask to a separate bitmap_zalloc() allocation
stored in buffer.scan_mask, but left iio_hw_buf_release() unchanged.

Free the scan mask in iio_hw_buf_release() before freeing the buffer
wrapper.