storvsc: use cmd_size to allocate per-command data

STORVSC uses its own momory pool to manage device request data.  However,
the SCSI layer already has a mechanisim for allocating additional memory
for each command issued to device driver. This patch removes the memory
pool in STORVSC and makes it use SCSI layer to allocate memory for device
request data.

Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>