core/exec-invoke: call setpriority() after sched_setattr()

The nice value is part of struct sched_attr, and consequently invoking
sched_setattr() after setpriority() would clobber the nice value with
the default (as we are not setting it in struct sched_attr).

It would be best to combine both calls, but for now simply invoke
setpriority() after sched_setattr() to make sure Nice= remains effective
when used together with CPUSchedulingPolicy=.

(cherry picked from commit 711a157)
(cherry picked from commit b628d4d)
(cherry picked from commit 4994f15)
(cherry picked from commit c90ba5e)

Author: intelfx

src/core/execute.c

@@ -4578,14 +4578,6 @@ static int exec_child(
                 }
         }
 
-        if (context->nice_set) {
-                r = setpriority_closest(context->nice);
-                if (r < 0) {
-                        *exit_status = EXIT_NICE;
-                        return log_unit_error_errno(unit, r, "Failed to set up process scheduling priority (nice level): %m");
-                }
-        }
-
         if (context->cpu_sched_set) {
                 struct sched_attr attr = {
                         .size = sizeof(attr),
@@ -4601,6 +4593,14 @@ static int exec_child(
                 }
         }
 
+        if (context->nice_set) {
+                r = setpriority_closest(context->nice);
+                if (r < 0) {
+                        *exit_status = EXIT_NICE;
+                        return log_unit_error_errno(unit, r, "Failed to set up process scheduling priority (nice level): %m");
+                }
+        }
+
         if (context->cpu_affinity_from_numa || context->cpu_set.set) {
                 _cleanup_(cpu_set_reset) CPUSet converted_cpu_set = {};
                 const CPUSet *cpu_set;