xfs: check the ondisk space mapping behind a dquot

Each xfs_dquot object caches the file offset and daddr of the ondisk
block that backs the dquot.  Make sure these cached values are the same
as the bmapi data, and that the block state is written.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>