selftests/cgroup: Preserve CPU hotplug write errors

The cpuset partition root state selftest checks several CPU hotplug
transitions. If writing to a CPU online file fails, the helper still
runs pause afterwards and returns the status of pause instead of the
failed write.

This hides the real hotplug failure and can make later checks run
against expectations for a transition that never happened. Move the
write before the bookkeeping and return when it fails, so callers can
observe the hotplug error and the test does not record a CPU as offline
unless the offline operation actually succeeded.

Also change the O* command handler in set_ctrl_state() to use
"eval $COMM $REDIRECT" like all other handlers. The previous version
set COMM but still called write_cpu_online directly, bypassing the
redirect that captures stderr for error reporting.

Changes since v1:
 - Use eval $COMM $REDIRECT in the O* handler instead of calling
   write_cpu_online directly (Waiman Long)

Fixes: a8c52eba88 ("kselftest/cgroup: Add cpuset v2 partition root state test")
Signed-off-by: Rui Qi <qirui.001@bytedance.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/testing/selftests/cgroup/test_cpuset_prs.sh

@@ -526,6 +526,7 @@ write_cpu_online()
 	CPU=${1%=*}
 	VAL=${1#*=}
 	CPUFILE=//sys/devices/system/cpu/cpu${CPU}/online
+	echo $VAL > $CPUFILE || return 1
 	if [[ $VAL -eq 0 ]]
 	then
 		OFFLINE_CPUS="$OFFLINE_CPUS $CPU"
@@ -535,7 +536,6 @@ write_cpu_online()
 					sort | uniq -u)
 		}
 	fi
-	echo $VAL > $CPUFILE
 	pause 0.05
 }
 
@@ -603,7 +603,8 @@ set_ctrl_state()
 			eval $COMM $REDIRECT
 			;;
 		    O*) VAL=${CMD#?}
-			write_cpu_online $VAL
+			COMM="write_cpu_online $VAL"
+			eval $COMM $REDIRECT
 			;;
 		    T*) COMM="echo 0 > $TFILE"
 			eval $COMM $REDIRECT