[XFS] use pagevec lookups This reduces the time spend in the radix tree

lookups and avoids unessecary look roundtrips.

SGI-PV: 947118
SGI-Modid: xfs-linux-melb:xfs-kern:203823a

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