SG: Change sg_set_page() to take length and offset argument

Most drivers need to set length and offset as well, so may as well fold
those three lines into one.

Add sg_assign_page() for those two locations that only needed to set
the page, where the offset/length is set outside of the function context.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/drivers/scsi/osst.c b/drivers/scsi/osst.c
index 1c5c4b6..4652ad2 100644
--- a/drivers/scsi/osst.c
+++ b/drivers/scsi/osst.c
@@ -5256,8 +5256,7 @@
 
 		STbuffer->sg[0].offset = 0;
 		if (page != NULL) {
-		    sg_set_page(&STbuffer->sg[0], page);
-		    STbuffer->sg[0].length = b_size;
+		    sg_set_page(&STbuffer->sg[0], page, b_size, 0);
 		    STbuffer->b_data = page_address(page);
 		    break;
 		}
@@ -5285,8 +5284,7 @@
 			normalize_buffer(STbuffer);
 			return 0;
 		}
-		sg_set_page(&STbuffer->sg[segs], page);
-		STbuffer->sg[segs].length = (OS_FRAME_SIZE - got <= PAGE_SIZE / 2) ? (OS_FRAME_SIZE - got) : b_size;
+		sg_set_page(&STbuffer->sg[segs], page, (OS_FRAME_SIZE - got <= PAGE_SIZE / 2) ? (OS_FRAME_SIZE - got) : b_size, 0);
 		got += STbuffer->sg[segs].length;
 		STbuffer->buffer_size = got;
 		STbuffer->sg_segs = ++segs;