Passive serialization support in the pool allocator

[Jason Thorpe <thorpej%me.com@localhost>]

An #ifdef that Taylor added in the new DRM code was bugging me, because a similar situation already existed in the NetBSD kernel, whereby the type stability of a object’s backing memory (the iam existentium case being LWPs) was required for a passively-serialized weak reference to work.  The LWP pool cache had a hack to handle it, and the hack was propagated to the new DRM code under an “#ifdef __NetBSD__”.

I initially fixed this by adding a “pre-destruct” callback that could be optionally set for a pool cache, but after further discussion, it seemed like having direct knowledge of passive serialization synchronization points in the allocator was a better solution, so here is a diff that implements it for you review.

	https://www.netbsd.org/~thorpej/pool-pser-diff.txt

Please comment ASAP, because I want to ride a kernel version bump.

-- thorpej