SELinux: don't BUG if fs reuses a superblock

I (wrongly) assumed that nfs_xdev_get_sb() would not ever share a superblock
and so cloning mount options would always be correct.  Turns out that isn't
the case and we could fall over a BUG_ON() that wasn't a BUG at all.  Since
there is little we can do to reconcile different mount options this patch
just leaves the sb alone and the first set of options wins.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: James Morris <jmorris@namei.org>

security/selinux/hooks.c

@@ -760,13 +760,13 @@ static void selinux_sb_clone_mnt_opts(const struct super_block *oldsb,
 	 * this early in the boot process. */
 	BUG_ON(!ss_initialized);
 
-	/* this might go away sometime down the line if there is a new user
-	 * of clone, but for now, nfs better not get here... */
-	BUG_ON(newsbsec->initialized);
-
 	/* how can we clone if the old one wasn't set up?? */
 	BUG_ON(!oldsbsec->initialized);
 
+	/* if fs is reusing a sb, just let its options stand... */
+	if (newsbsec->initialized)
+		return;
+
 	mutex_lock(&newsbsec->lock);
 
 	newsbsec->flags = oldsbsec->flags;