This adds filemap_write_and_wait_range(mapping, lstart, lend) which starts
writeback and waits on a range of pages.  We call this from
__blkdev_direct_IO with just the range that is going to be read by the
direct_IO read.  It was lightly tested with fsx and ext3 and passed.
Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>