xfs: treat idx as a cursor in xfs_bmap_del_extent_*

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

@@ -4750,12 +4750,12 @@ xfs_bmap_del_extent_delay(
 						       del->br_blockcount);
 
 		got->br_startblock = nullstartblock((int)got_indlen);
-		xfs_iext_update_extent(ip, state, *idx, got);
 
 		new.br_startoff = del_endoff;
 		new.br_state = got->br_state;
 		new.br_startblock = nullstartblock((int)new_indlen);
 
+		xfs_iext_update_extent(ip, state, *idx, got);
 		++*idx;
 		xfs_iext_insert(ip, *idx, 1, &new, state);
 
@@ -4832,13 +4832,13 @@ xfs_bmap_del_extent_cow(
 		 * Deleting the middle of the extent.
 		 */
 		got->br_blockcount = del->br_startoff - got->br_startoff;
-		xfs_iext_update_extent(ip, state, *idx, got);
 
 		new.br_startoff = del_endoff;
 		new.br_blockcount = got_endoff - del_endoff;
 		new.br_state = got->br_state;
 		new.br_startblock = del->br_startblock + del->br_blockcount;
 
+		xfs_iext_update_extent(ip, state, *idx, got);
 		++*idx;
 		xfs_iext_insert(ip, *idx, 1, &new, state);
 		break;
@@ -5054,8 +5054,8 @@ xfs_bmap_del_extent_real(
 			flags |= xfs_ilog_fext(whichfork);
 		XFS_IFORK_NEXT_SET(ip, whichfork,
 			XFS_IFORK_NEXTENTS(ip, whichfork) + 1);
-		xfs_iext_insert(ip, *idx + 1, 1, &new, state);
 		++*idx;
+		xfs_iext_insert(ip, *idx, 1, &new, state);
 		break;
 	}