selftests/resctrl: Check for resctrl mount point only if resctrl FS is supported

check_resctrlfs_support() does the following
1. Checks if the platform supports resctrl file system or not by looking
for resctrl in /proc/filesystems
2. Calls opendir() on default resctrl file system path
(i.e. /sys/fs/resctrl)
3. Checks if resctrl file system is mounted or not by looking at
/proc/mounts

Steps 2 and 3 will fail if the platform does not support resctrl file
system. So, there is no need to check for them if step 1 fails.

Fix this by returning immediately if the platform does not support
resctrl file system.

Tested-by: Babu Moger <[e-mail]>
Signed-off-by: Fenghua Yu <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/resctrl/resctrlfs.c

···
 	ksft_print_msg("%s kernel supports resctrl filesystem\n",
 		       ret ? "Pass:" : "Fail:");
 
+	if (!ret)
+		return ret;
+
 	dp = opendir(RESCTRL_PATH);
 	ksft_print_msg("%s resctrl mountpoint \"%s\" exists\n",
 		       dp ? "Pass:" : "Fail:", RESCTRL_PATH);