vfs: new helper - vfs_ustat()

... and bury user_get_super()/statfs_by_dentry() - they are
purely internal now.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/compat.c b/fs/compat.c
index c987875..9db5a60 100644
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -342,16 +342,9 @@
  */
 asmlinkage long compat_sys_ustat(unsigned dev, struct compat_ustat __user *u)
 {
-	struct super_block *sb;
 	struct compat_ustat tmp;
 	struct kstatfs sbuf;
-	int err;
-
-	sb = user_get_super(new_decode_dev(dev));
-	if (!sb)
-		return -EINVAL;
-	err = statfs_by_dentry(sb->s_root, &sbuf);
-	drop_super(sb);
+	int err = vfs_ustat(new_decode_dev(dev), &sbuf);
 	if (err)
 		return err;