[XFS] Update XFS Documentation for ikeep and ihashsize

Update xfs docs for:
* In memory inode hashes has been removed.
* noikeep is now the default.

SGI-PV: 969561
SGI-Modid: 2.6.x-xfs-melb:linux:29481b

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tim Shimmin <tes@sgi.com>