scsi: storvsc: Don't assume that the scatterlist is not chained

The current code assumes that the scatterlists presented are not chained.
Fix the code to not make this assumption.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>