pass a struct path to vfs_statfs

We'll need the path to implement the flags field for statvfs support.
We do have it available in all callers except:

 - ecryptfs_statfs.  This one doesn't actually need vfs_statfs but just
   needs to do a caller to the lower filesystem statfs method.
 - sys_ustat.  Add a non-exported statfs_by_dentry helper for it which
   doesn't won't be able to fill out the flags field later on.

In addition rename the helpers for statfs vs fstatfs to do_*statfs instead
of the misleading vfs prefix.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ecryptfs/super.c

@@ -118,7 +118,11 @@ void ecryptfs_init_inode(struct inode *inode, struct inode *lower_inode)
  */
 static int ecryptfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 {
-	return vfs_statfs(ecryptfs_dentry_to_lower(dentry), buf);
+	struct dentry *lower_dentry = ecryptfs_dentry_to_lower(dentry);
+
+	if (!lower_dentry->d_sb->s_op->statfs)
+		return -ENOSYS;
+	return lower_dentry->d_sb->s_op->statfs(lower_dentry, buf);
 }
 
 /**