Newly allocated memory is subtracted from inuse, while it was never added to inuse.
Span leftovers are subtracted from both inuse and idle,
while they were never added.
Fixes #8544.
Fixes #8430.

LGTM=khr, cookieo9
R=golang-codereviews, khr, cookieo9
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/130200044