selftests/ftrace: Fix to use remount when testing mount GID option

Fix mount_options.tc to use remount option to mount the tracefs.
Since the current implementation does not umount the tracefs,
this test always fails because of -EBUSY error.
Using remount option will allow us to change the mount option.

Link: https://lore.kernel.org/r/173625186741.1383744.16707876180798573039.stgit@devnote2
Fixes: 8b55572e5180 ("tracing/selftests: Add tracefs mount options test")
Signed-off-by: Masami Hiramatsu (Google) <[e-mail]>
Cc: [e-mail]
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/ftrace/test.d/00basic/mount_options.tc

···
 	tac /etc/group | grep -v ":$original_gid:" | head -1 | cut -d: -f3
 }
 
-mount_tracefs_with_options() {
+remount_tracefs_with_options() {
 	local mount_point="$1"
 	local options="$2"
 
-	mount -t tracefs -o "$options" nodev "$mount_point"
+	mount -t tracefs -o "remount,$options" nodev "$mount_point"
 
 	setup
 }
···
 
 	# Unmount existing tracefs instance and mount with new GID
 	unmount_tracefs "$mount_point"
-	mount_tracefs_with_options "$mount_point" "$new_options"
+	remount_tracefs_with_options "$mount_point" "$new_options"
 
 	check_gid "$mount_point" "$other_group"
 
···
 
 	# Unmount and remount with the original GID
 	unmount_tracefs "$mount_point"
-	mount_tracefs_with_options "$mount_point" "$mount_options"
+	remount_tracefs_with_options "$mount_point" "$mount_options"
 	check_gid "$mount_point" "$original_group"
 }