[XFS] Account for the page we just wrote when we detect congestion during

the clustering of extra pages in a buffered write.

SGI-PV: 949210
SGI-Modid: xfs-linux-melb:xfs-kern:25130a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>