Use xfs_iext_get_extent to find, and xfs_iext_update_extent to update
entries in the in-core extent list. This isolates the function from
the detailed layout of the extent list, and generally makes the code
a lot more readable.
Also get rid of the oldext and newext variables as using the extent
records is a lot more descriptive.
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>