xfs: treat idx as a cursor in xfs_bmap_add_extent_delay_real

xfs: treat idx as a cursor in xfs_bmap_add_extent_delay_real

Source kernel commit: 0d045540ed57e68b9ddcf5c325279a7e5ede7bbf

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>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>