This patch registers the ARC shrinker by using the event handler
list from BSD side. When ARC is initialized, it inserts the lowmem event
handler into an external event handler list. lowmem basically signals
the reclaiming thread which will then wake up to decide which approach
should be used to shrink the ARC.

The memory pressure on OSv is activated when the 20% watermark is
reached, so the shrink policy will decide which shrinker should
be called on such events.

bsd_shrinker_init is the responsible to find the lowmem event handler
from the external list, and integrate it into our shrinker infrastructure.

arc_lowmem needed few changes to return the amount of released memory
from the ARC.

Glauber and I tested the functionality by filling up the ARC up
to its target, then allocating as much memory as possible to see
if the ARC shrinker would take place to release memory back to
the operating system.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>