arc: Re-enable portions of code needed by reclaim thread

arc_reclaim_needed checks if reclaiming is needed by looking
at two possible conditions:
1) the variable needfree is set.
2) memory used is higher than 3/4 of the system memory.

The actual problem is that arc_reclaim_needed was completely
disabled. It would always return 0, i.e. reclaim not needed.

Add stub to vm_paging_needed to avoid an unneeded ifdef.

Reclaiming support is planned, so let's re-enable what's needed
and implement kmem_used which is used to determine the amount
of memory used so far.

Reviewed-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>