Fix freeing of channels and buffers in buffers.c v2

Changes since v1:
Handle new naming conventions

buffers.c appeared to be very convoluted full of krefs and in
general very bad shape. I have tried to make the freeing and
unmapping of shared memory symmetrical in the sense that every
open has a corresponding close and every map has a corresponding
unmap.

I have removed all use of krefs. Assuming no concurrency this
should be fine. The kref usage was anyway broken.

Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>