xfs: correct calculation for agend and blockcount

The agend should be "start + length - 1", then, blockcount should be
"end + 1 - start".  Correct 2 calculation mistakes.

Also, rename "agend" to "range_agend" because it's not the end of the AG
per se; it's the end of the dead region within an AG's agblock space.

Fixes: 5cf32f63 ("xfs: fix the calculation for "end" and "length"")
Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

fs/xfs/xfs_notify_failure.c

@@ -126,8 +126,8 @@ xfs_dax_notify_ddev_failure(
 		struct xfs_rmap_irec	ri_low = { };
 		struct xfs_rmap_irec	ri_high;
 		struct xfs_agf		*agf;
-		xfs_agblock_t		agend;
 		struct xfs_perag	*pag;
+		xfs_agblock_t		range_agend;
 
 		pag = xfs_perag_get(mp, agno);
 		error = xfs_alloc_read_agf(pag, tp, 0, &agf_bp);
@@ -148,10 +148,10 @@ xfs_dax_notify_ddev_failure(
 			ri_high.rm_startblock = XFS_FSB_TO_AGBNO(mp, end_fsbno);
 
 		agf = agf_bp->b_addr;
-		agend = min(be32_to_cpu(agf->agf_length),
+		range_agend = min(be32_to_cpu(agf->agf_length) - 1,
 				ri_high.rm_startblock);
 		notify.startblock = ri_low.rm_startblock;
-		notify.blockcount = agend - ri_low.rm_startblock;
+		notify.blockcount = range_agend + 1 - ri_low.rm_startblock;
 
 		error = xfs_rmap_query_range(cur, &ri_low, &ri_high,
 				xfs_dax_failure_fn, &notify);