loop: factor out a loop_assign_backing_file helper

Split the code for setting up a backing file into a helper in preparation
of adding more code to this path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250131120120.1315125-2-hch@lst.de


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