Propagate unencrypted_size up to the file cache root
Return unencrypted_size of folder when queried

This fixes the "used space" to be based on the unencrypted size, not
encrypted size, to be consistent with how quota/space is handled when
encryption is enabled
Merge pull request #7624 from owncloud/enc-encryptedusedspacefix

[master] Fixed used space to be based on unencrypted size