The new workaround list allocator grows the list in chunks so will end up
with some unused space. Trim it when the initialization phase is done to
free up a tiny bit of slab.

v2:
 * Simplify with kmemdup. (Chris Wilson)

v3:
 * Refactor for __size removal.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203125014.3219-8-tvrtko.ursulin@linux.intel.com