[XFS] Fix inode list allocation size in writeback.

We only need to allocate space for the number of inodes in the cluster
when writing back inodes, not every byte in the inode cluster. This
reduces the amount of memory needing to be allocated to 256 bytes instead
of 64k.

SGI-PV: 981949
SGI-Modid: xfs-linux-melb:xfs-kern:31182a

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