gfs2: implement gfs2_block_zero_range using iomap_zero_range

iomap handles all the nitty-gritty details of zeroing a file
range for us, so use the proper helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>