dlm: prevent NPD when writing a positive value to event_done

In the Linux kernel, the following vulnerability has been resolved: dlm: prevent NPD when writing a positive value to event_done do_uevent returns the value written to event_done. In case it is a positive value, new_lockspace would undo all the work, and lockspace would not be set. __dlm_new_lockspace, however, would treat that positive value as a success due to commit 8511a2728ab8 ("dlm: fix use count with multiple joins"). Down the line, device_create_lockspace would pass that NULL lockspace to dlm_find_lockspace_local, leading to a NULL pointer dereference. Treating such positive values as successes prevents the problem. Given this has been broken for so long, this is unlikely to break userspace expectations.