staging:iio: Fix sw_ring memory corruption

The sw_ring does not properly handle the case where the write pointer already
has wrapped around, the read pointer has not and the remaining buffer space at
the end is enough to fill the read buffer:

  +-----------------------------------+
  |     |              |##data##|     |
  +-----------------------------------+
     write_p        read_p

In this case the current code will copy all available data to the buffer and
as a result will write beyond the bounds of the buffer and cause a memory
corruption.

To address this issue this patch adds code to calculate the available buffer
space and makes sure that the number of bytes to copy does not exceed this
number. This allows the code which copies the data around to be simplified as
it only has to consider two cases: Read wraps around and read does not wrap
around.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>