configfs, securityfs: kill_litter_super() not needed

configfs, securityfs: kill_litter_super() not needed

These are guaranteed to be empty by the time they are shut down;
both are single-instance and there is an internal mount maintained
for as long as there is any contents.

Both have that internal mount pinned by every object in root.

In other words, kill_litter_super() boils down to kill_anon_super()
for those.

Reviewed-by: Joel Becker <jlbec@evilplan.org>
Acked-by: Paul Moore <paul@paul-moore> (LSM)
Acked-by: Andreas Hindborg <a.hindborg@kernel.org> (configfs)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>