[XFS] only mark buffers done when all pages are uptodate in addition

replace PBF_NONE with an inverted PBF_DONE, so it's like all the other
flags.

SGI-PV: 942609
SGI-Modid: xfs-linux:xfs-kern:199136a

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