xfs: treat idx as a cursor in xfs_bmap_add_extent_unwritten_real

xfs: treat idx as a cursor in xfs_bmap_add_extent_unwritten_real

Source kernel commit: a681847796df31c036810bce97003f687e1db5f4

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>