[XFS] cluster rewrites We can cluster mapped pages aswell, this improves

performances on rewrites since we can reduce the number of allocator
calls.

SGI-PV: 947118
SGI-Modid: xfs-linux-melb:xfs-kern:203829a

Signed-off-by: Christoph Hellwig <hch@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>