[XFS] Increase the size of the buffer holding the local inode cluster

list, to increase our potential readahead window and in turn improve
bulkstat performance.

SGI-PV: 944409
SGI-Modid: xfs-linux-melb:xfs-kern:26607a

Signed-off-by: Nathan Scott <nathans@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>