BACKPORT: loop: Rework lo_ioctl() __user argument casting

In preparation for a new ioctl that needs to copy_from_user(); makes the
code easier to read as well.

Signed-off-by: Martijn Coenen <maco@android.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 571fae6e290d64a3e8132c455e7786c99c467ed1)
Bug: 148607611
Change-Id: Ie37d0ed6bfc91d2a55c070446e5c54c8343a7170