null_blk: fix validation of block size

In the Linux kernel, the following vulnerability has been resolved: null_blk: fix validation of block size Block size should be between 512 and PAGE_SIZE and be a power of 2. The current check does not validate this, so update the check. Without this patch, null_blk would Oops due to a null pointer deref when loaded with bs=1536 [1]. [axboe: remove unnecessary braces and != 0 check]