xfs: treat idx as a cursor in xfs_bmap_add_extent_hole_real

Stop poking before and after the index and just increment or decrement
it while doing our operations on it to prepare for a new extent list
implementation.
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>

fs/xfs/libxfs/xfs_bmap.c

@@ -2742,11 +2742,11 @@ xfs_bmap_add_extent_hole_real(
 		 * left and on the right.
 		 * Merge all three into a single extent record.
 		 */
-		--*idx;
 		left.br_blockcount += new->br_blockcount + right.br_blockcount;
-		xfs_iext_update_extent(ip, state, *idx, &left);
 
-		xfs_iext_remove(ip, *idx + 1, 1, state);
+		xfs_iext_remove(ip, *idx, 1, state);
+		--*idx;
+		xfs_iext_update_extent(ip, state, *idx, &left);
 
 		XFS_IFORK_NEXT_SET(ip, whichfork,
 			XFS_IFORK_NEXTENTS(ip, whichfork) - 1);
@@ -2778,10 +2778,10 @@ xfs_bmap_add_extent_hole_real(
 		 * on the left.
 		 * Merge the new allocation with the left neighbor.
 		 */
-		--*idx;
 		old = left;
-
 		left.br_blockcount += new->br_blockcount;
+
+		--*idx;
 		xfs_iext_update_extent(ip, state, *idx, &left);
 
 		if (cur == NULL) {