libos: a few fixes in kernel sbrk
libos: a few fixes in kernel sbrk

- Lock check was inverted (which would livelock)
- Add a check for heap exhaustion

jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor