buffers: bring back cache occupancy improvement

buffers: bring back cache occupancy improvement

The improvement was removed in 40129,
causing 5-40% regressions in AVF tests.

There is a memory-speed trade-off,
this change prefers speed over memory efficiency.

Ideally, the choice should be configurable,
but that is not easy to achieve, considering
how early is vlib_buffer_main_init called.

Type: fix
Fixes: 038dad7ef29b0b724071edb5f8cc7a9845584454

Change-Id: I4746f3634abe6d233c9d092a372de05b3d1ae4b6
Signed-off-by: Vratko Polak <vrpolak@cisco.com>