xfs: refactor xfs_del_extent_real

Use 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.

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>