block: Add BLKROTATIONAL ioctl

Introduce an ioctl which permits applications to query whether a block
device is rotational.

Change-Id: If1ebe4e64b1e8aee9b9304cf086cb982933f8cb3
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>