loop: Use blk_validate_block_size() to validate block size

Remove loop_validate_block_size() and use the block layer helper
to validate block size.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20211026144015.188-4-xieyongji@bytedance.com


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