loop: open code the direct I/O flag update in loop_set_dio

loop_set_dio is different from the other (__)loop_update_dio callers in
that it doesn't take any implicit conditions into account and wants to
update the direct I/O flag to the user passed in value and fail if that
can't be done.

Open code the logic here to prepare for simplifying the other direct I/O
flag updates and to make the error handling less convoluted.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250110073750.1582447-6-hch@lst.de


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