scsi: Use extract_iter_to_sg()

Use extract_iter_to_sg() to build a scatterlist from an iterator.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: James E.J. Bottomley <jejb@linux.ibm.com>
cc: Martin K. Petersen <martin.petersen@oracle.com>
cc: Christoph Hellwig <hch@lst.de>
cc: linux-scsi@vger.kernel.org