loop: split loop_lookup

loop_lookup has two callers - one wants to do the a find by index and the
other wants any unbound loop device.  Open code the respective
functionality in each caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210623145908.92973-9-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>