We reuse finalizers in finblocks, which are allocated off-heap. This
means they have to be zero-initialized before becoming visible to the
garbage collector. We actually already do this by clearing the
finalizer before returning it to the pool, but we're not careful to
enforce correct memory ordering. Fix this by manipulating the
finalizer count atomically so these writes synchronize properly with
the garbage collector.

Updates #17503.

Change-Id: I7797d31df3c656c9fe654bc6da287f66a9e2037d
Reviewed-on: https://go-review.googlesource.com/31454Reviewed-by: Rick Hudson <rlh@golang.org>