xfs: simplify xfs_bmap_punch_delalloc_range

Instead of using xfs_bmapi_read to find delalloc extents and then punch
them out using xfs_bunmapi, opencode the loop to iterate over the extents
and call xfs_bmap_del_extent_delay directly.  This both simplifies the
code and reduces the number of extent tree lookups required.

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>