core/unit: do not load drop-in configs for masked unit files

Otherwise, ExecXYZ= given by drop-ins may be added to a masked unit,
then commands may be unexpectedly executed. For example, if an ExecStop=
is set in a drop-in for a running unit, then 'systemctl mask --now' for
the unit try to invoke the command specified in ExecStop=.

Fixes #38802.

src/core/unit.c

@@ -1422,6 +1422,9 @@ int unit_load_fragment_and_dropin(Unit *u, bool fragment_required) {
         if (r < 0)
                 return r;
 
+        if (u->load_state == UNIT_MASKED)
+                return 0;
+
         if (u->load_state == UNIT_STUB) {
                 if (fragment_required)
                         return -ENOENT;