no explicit xfs_iflush for special inodes during unmount

Currently we explicitly call xfs_iflush on the quota, real-time and root
inodes from xfs_unmount_flush.  But we just called xfs_sync_inodes with
SYNC_ATTR and do an XFS_bflush aka xfs_flush_buftarg to make sure all inodes
are on disk already, so there is no need for these special cases.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>