tools/sched_ext: fix strtoul() misuse in scx_hotplug_seq()

scx_hotplug_seq() uses strtoul() but validates the result with a
negative check (val < 0), which can never be true for an unsigned
return value.

Use the endptr mechanism to verify the entire string was consumed,
and check errno == ERANGE for overflow detection.

Signed-off-by: David Carlier <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

tools/sched_ext/include/scx/compat.h

···
 {
 	int fd;
 	char buf[32];
+	char *endptr;
 	ssize_t len;
 	long val;
 
···
 	buf[len] = 0;
 	close(fd);
 
-	val = strtoul(buf, NULL, 10);
-	SCX_BUG_ON(val < 0, "invalid num hotplug events: %lu", val);
+	errno = 0;
+	val = strtoul(buf, &endptr, 10);
+	SCX_BUG_ON(errno == ERANGE || endptr == buf ||
+		   (*endptr != '\n' && *endptr != '\0'), "invalid num hotplug events: %ld", val);
 
 	return val;
 }