loop: Consolidate size calculation logic into lo_calculate_size()

Renamed get_size to lo_calculate_size and merged the logic from get_size
and get_loop_size into a single function. Update all callers to use
lo_calculate_size. This is done in preparation for improving the size
detection logic.

Signed-off-by: Rajeev Mishra <rajeevm@hpe.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250818184821.115033-2-rajeevm@hpe.com


[axboe: massage commit message]
Signed-off-by: Jens Axboe <axboe@kernel.dk>