xfs: remove the xfs_disk_dquot_t and xfs_dquot_t

Signed-off-by: Pavel Reichl <preichl@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: fix some of the comments]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>