quota: Wire up Q_GETXSTATE and Q_GETXSTATV calls to work with ->get_state

Add appropriate conversion functions so that filesystems supporting
->get_state() method can be queried using Q_GETXSTATE and Q_GETXSTATV
calls.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>