[XFS] Use the incore inode size in xfs_file_readdir()

We should be using the incore inode size here not the linux inode
size.  The incore inode size is always up to date for directories
whereas the linux inode size is not updated for directories.

We've hit assertions in xfs_bmap() and traced it back to the linux
inode size being zero but the incore size being correct.

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