ext4: add unmount filesystem message

Now that we have kernel message at mount time, system administrator
could acquire the mount time, device and options easily. But we don't
have corresponding unmounting message at umount time, so we cannot know
if someone umount a filesystem easily. Some of the modern filesystems
(e.g. xfs) have the umounting kernel message, so add one for ext4
filesystem for convenience.

 EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none.
 EXT4-fs (sdb): unmounting filesystem.
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220412145320.2669897-1-yi.zhang@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -1211,6 +1211,9 @@ static void ext4_put_super(struct super_block *sb)
 	 */
 	ext4_unregister_sysfs(sb);
 
+	if (___ratelimit(&ext4_mount_msg_ratelimit, "EXT4-fs unmount"))
+		ext4_msg(sb, KERN_INFO, "unmounting filesystem.");
+
 	ext4_unregister_li_request(sb);
 	ext4_quota_off_umount(sb);