xfs: remove xfs_iread

There is not much point in the xfs_iread function, as it has a single
caller and not a whole lot of code.  Move it into the only caller,
and trim down the overdocumentation to just documenting the important
"why" instead of a lot of redundant "what".

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>