splice: Make splice from an O_DIRECT fd use copy_splice_read()

Make a read splice from a file descriptor that's open O_DIRECT use
copy_splice_read() to do the reading as filemap_splice_read() is unlikely
to find any pagecache to splice.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Jens Axboe <axboe@kernel.dk>
cc: linux-fsdevel@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/20230522135018.2742245-8-dhowells@redhat.com


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