[XFS] Implement fallocate.

Implement the new generic callout for file preallocation. Atomically
change the file size if requested.

SGI-PV: 972756
SGI-Modid: xfs-linux-melb:xfs-kern:30009a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>