kill xfs_unmount_flush

There's almost nothing left in this function, instead remove the IRELE
on the real times inodes and the call to XFS_QM_UNMOUNT into xfs_unmountfs.

For the regular unmount case that means it now also happenes after dmapi
notification, but otherwise there is no difference in behaviour.

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