kmem_cache: Call constructor/destructor on each _alloc and _free

This has a few benefits. First, it fixes a regression that occurred in
splat's slab_reap and slab_age tests causd by "Rework generic memory
allocation interfaces". Second, it makes porting code from Illumos to
ZFSOnLinux easier. Third, it has the side effect of making reclaim from
slab caches that specify reclaim functions an order of magnitude faster.
The splat slab_reap test usually took 30 to 40 seconds. With this
change, it takes 3 to 4.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Remove kmem_alloc_node interface

This interface has no external consumers and does not exist on Solaris.
Lets remove it.

Signed-off-by: Richard Yao <ryao@gentoo.org>