runtime: fix stack split detection around fork If runtime_BeforeFork splits stack, it will unsplit it with spoiled g->stackguard. It leads to check failure in oldstack:
runtime: fix stack split detection around fork
If runtime_BeforeFork splits stack, it will unsplit it
with spoiled g->stackguard. It leads to check failure in oldstack:

fatal error: stackfree: bad fixed size

runtime stack:
runtime.throw(0xadf3cd)
runtime.stackfree(0xc208040480, 0xfffffffffffff9dd, 0x1b00fa8)
runtime.oldstack()
runtime.lessstack()

goroutine 311 [stack unsplit]:
syscall.forkAndExecInChild(0xc20802eea0, 0xc208192c00, 0x5, 0x5, 0xc208072a80, ...)
syscall.forkExec(0xc20802ed80, 0x54, 0xc2081ccb40, 0x4, 0x4, ...)

Fixes issue 7567.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, khr, rsc
https://codereview.appspot.com/77340045

firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old