oomd: Return tristate status from oomd_cgroup_kill_mark()

oomd: Return tristate status from oomd_cgroup_kill_mark()

oomd_cgroup_kill_mark() currently returns 0 on all non-error paths. But
the manager only logs that it marked for killing on `if (r > 0)`, which
is thus unreachable.

Changing it to `r >= 0` would also be wrong, because then we would log
on no-op paths.

So let's fix this by making the return value express what actually
happened:

- < 0: failure to queue the kill state
-   0: no new mark was created (already queued or dry-run)
- > 0: a new kill state was queued