fix oops in fs/9p late mount failure

if 9P ->get_sb() fails late (at root inode or root dentry
allocation), we'll hit its ->kill_sb() with NULL ->s_root
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/9p/vfs_super.c

@@ -188,7 +188,8 @@ static void v9fs_kill_super(struct super_block *s)
 
 	P9_DPRINTK(P9_DEBUG_VFS, " %p\n", s);
 
-	v9fs_dentry_release(s->s_root);	/* clunk root */
+	if (s->s_root)
+		v9fs_dentry_release(s->s_root);	/* clunk root */
 
 	kill_anon_super(s);