Merge xfs_iextract and xfs_idestroy into xfs_ireclaim as they are never
called individually.  Also rewrite most comments in this area as they
were severly out of date.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDave Chinner <david@fromorbit.com>
Signed-off-by: NNiv Sardi <xaiki@sgi.com>