slurm init script : bug correction in stop target error code mgmt

In current version of slurm initscript, a stop action returns a
non null exit code as slurmstatus exit code is directly used and
the daemons are stopped.
Ensure that when called from slurmstop, slurmstatus error code is
reversed to correctly match the attended error code of the stop
stage.
Port of v2.4 commit a09bffa5
Matthieu Hautreux authored 3 months ag