[XFS] Check that a page has dirty buffers before finding it acceptable for

rewrite clustering. This prevents writing excessive amounts of clean data
when doing random rewrites of a cached file.

SGI-PV: 951193
SGI-Modid: xfs-linux-melb:xfs-kern:25531a

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