mm: use filemap_range_needs_writeback() for O_DIRECT reads

mm: use filemap_range_needs_writeback() for O_DIRECT reads

For the generic page cache read helper, use the better variant of checking
for the need to call filemap_write_and_wait_range() when doing O_DIRECT
reads. This avoids falling back to the slow path for IOCB_NOWAIT, if there
are no pages to wait for (or write out).

Signed-off-by: Jens Axboe <axboe@kernel.dk>