loop: only write back pagecache when starting to to use direct I/O

There is no point in doing an fdatasync to write out pages when switching
away from direct I/O, as there won't be any.  The writeback is only
needed when switching to direct I/O, which would have to invalidate the
pagecache less efficiently from the I/O path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250110073750.1582447-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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