sysfs-tagging.txt: fix pre-kernfs references

 - sysfs_dirent is now kernfs_node - see commit 324a56e1 ("kernfs:
   s/sysfs_dirent/kernfs_node/ and rename its friends accordingly")

 - sysfs_super_info is now kernfs_super_info - see commit c525aadd
   ("kernfs: s/sysfs/kernfs/ in various data structures")

 - the 's_' prefix was dropped from various fields - see
   commit adc5e8b5 ("kernfs: drop s_ prefix from kernfs_node members")

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>