selinux: fix sel_kill_sb()

Fix the order in sel_kill_sb() to match other pseudo filesystems.
This does not currently matter for selinuxfs per se but could
in the future, e.g. with SELinux namespaces and multiple selinuxfs
instances.

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>