cpupower: monitor: Exit with error status if execvp() fail

In the case that we give a invalid command to idle_monitor for
monitoring, the execvp() will fail and thus go to the next line.
As a result, we'll see two differnt monitoring output. For
example, running `cpupower monitor -i 5 invalidcmd` which `invalidcmd`
is not executable.

Link: https://lore.kernel.org/r/20250220163846.2765-1-s921975628@gmail.com
Signed-off-by: Yiwei Lin <s921975628@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: RinHizakura

tools/power/cpupower/utils/idle_monitor/cpupower-monitor.c

@@ -6,6 +6,7 @@
  */
 
 
+#include <errno.h>
 #include <stdio.h>
 #include <unistd.h>
 #include <stdlib.h>
@@ -294,7 +295,10 @@ int fork_it(char **argv)
 
 	if (!child_pid) {
 		/* child */
-		execvp(argv[0], argv);
+		if (execvp(argv[0], argv) == -1) {
+			printf("Invalid monitor command %s\n", argv[0]);
+			exit(errno);
+		}
 	} else {
 		/* parent */
 		if (child_pid == -1) {