xfs: also call xfs_file_iomap_end_delalloc for zeroing operations

There is no reason not to punch out stale delalloc blocks for zeroing
operations, as they otherwise behave exactly like normal writes.

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