xfs: fix extent busy updating

In xfs_extent_busy_update_extent() case 6 and 7, whenever bno is modified on
extent busy, the relavent length has to be modified accordingly.

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>