selinux: fix missing dput() before selinuxfs unmount

Commit 0619f0f5 ("selinux: wrap selinuxfs state") triggers a BUG
when SELinux is runtime-disabled (i.e. systemd or equivalent disables
SELinux before initial policy load via /sys/fs/selinux/disable based on
/etc/selinux/config SELINUX=disabled).

This does not manifest if SELinux is disabled via kernel command line
argument or if SELinux is enabled (permissive or enforcing).

Before:
  SELinux:  Disabled at runtime.
  BUG: Dentry 000000006d77e5c7{i=17,n=null}  still in use (1) [unmount of selinuxfs selinuxfs]

After:
  SELinux:  Disabled at runtime.

Fixes: 0619f0f5 ("selinux: wrap selinuxfs state")
Reported-by: NTetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Reported-by: NDmitry Vyukov <dvyukov@google.com>
Signed-off-by: NStephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

security/selinux/selinuxfs.c

@@ -2061,6 +2061,7 @@ __initcall(init_sel_fs);
 void exit_sel_fs(void)
 {
 	sysfs_remove_mount_point(fs_kobj, "selinux");
+	dput(selinux_null.dentry);
 	kern_unmount(selinuxfs_mount);
 	unregister_filesystem(&sel_fs_type);
 }