[release-branch.go1.9] runtime: in cpuProfile.addExtra, set p.lostExtra to 0 after flush
After the number of lost extra events are written to the the cpuprof log,
the number of lost extra events should be set to zero, or else, the next
time time addExtra is logged, lostExtra will be overcounted. This change
resets lostExtra after its value is written to the log.