gocore: distinguish released memory

Split up the free span space into 2 categories, the space which we
have released back to the OS vs the space we haven't.

Change-Id: I897c57a10e5cb4ab7321711fece8addcf5f4ab19
Reviewed-on: https://go-review.googlesource.com/92417
Reviewed-by: Austin Clements <austin@google.com>
1 file changed