loop: set LO_FLAGS_DIRECT_IO in loop_assign_backing_file

Assigning LO_FLAGS_DIRECT_IO from the O_DIRECT flag is related to
assigning a new backing file.  Move the assignment in preparation
of using the flag more and earlier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250131120120.1315125-3-hch@lst.de


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