xfs: simplify sector number calculation in xfs_zero_extent

xfs_zero_extent does some really odd gymnstics to calculate the block
layer sectors numbers passed to blkdev_issue_zeroout.  This is because it
used to call sb_issue_zeroout and the calculations in that helper got
open coded here in the rather misleadingly named commit 3dc29161
("dax: use sb_issue_zerout instead of calling dax_clear_sectors").

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>