[media] media: dvb_ringbuffer: Add memory barriers

[media] media: dvb_ringbuffer: Add memory barriers

Implement memory barriers according to Documentation/circular-buffers.txt:
- use smp_store_release() to update ringbuffer read/write pointers
- use smp_load_acquire() to load write pointer on reader side
- use ACCESS_ONCE() to load read pointer on writer side

This fixes data stream corruptions observed e.g. on an ARM Cortex-A9
quad core system with different types (PCI, USB) of DVB tuners.

Signed-off-by: Soeren Moch <smoch@web.de>
Cc: stable@vger.kernel.org # 3.14+
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>