Fix return value from ng_uncallout().

Fix return value from ng_uncallout().

callout_stop() recently started returning -1 when the callout is already
stopped, which is not handled by the netgraph code. Properly filter
the return value. Netgraph callers only want to know if the callout
was cancelled and not draining or already stopped.

Discussed with:		julian, glebius
MFC after:		2 weeks