Staging: IIO: Ring buffer: Initial pass at rarely locked ring buffer

Please note this ring buffer implementation is very much a
work in progress (and hence RFC).  In it's current form
it is stable and reasonably efficient.  There are a couple
of unlikely cases that will lead to more data being lost
that is strictly necessary. The target was for the case
of requiring regular sampling even during user space reads.

All comments welcome.

The intention is to make this only one of several
implementations with run time selection.  For now there
is only one, so it is hard coded into the drivers using it.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>