[release-branch.go1.1] runtime: fix heap corruption during GC

««« CL 9831043 / e84e7204b01b
runtime: fix heap corruption during GC
The 'n' variable is used during rescan initiation in GC_END case,
but it's overwritten with chan capacity in GC_CHAN case.
As the result rescan is done with the wrong object size.
Fixes #5554.

R=golang-dev, khr
CC=golang-dev
https://golang.org/cl/9831043
»»»

R=dvyukov, khr, dave
CC=golang-dev
https://golang.org/cl/10028044