staging: most: add fair buffer distribution

This patch ensures a fair distribution of buffers, when two AIMs share a
single channel. The AIMs then won't be able to use more than half of all
pre-allocated buffers of the linked channel. However, in case the channel
is not shared, the AIM can exclusively use all available buffers.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>