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/md/dm-crypt.c b/drivers/md/dm-crypt.c
index ac54f69..1c159ac 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -351,14 +351,10 @@
 		struct scatterlist sg_in, sg_out;
 
 		sg_init_table(&sg_in, 1);
-		sg_set_page(&sg_in, bv_in->bv_page);
-		sg_in.offset = bv_in->bv_offset + ctx->offset_in;
-		sg_in.length = 1 << SECTOR_SHIFT;
+		sg_set_page(&sg_in, bv_in->bv_page, 1 << SECTOR_SHIFT, bv_in->bv_offset + ctx->offset_in);
 
 		sg_init_table(&sg_out, 1);
-		sg_set_page(&sg_out, bv_out->bv_page);
-		sg_out.offset = bv_out->bv_offset + ctx->offset_out;
-		sg_out.length = 1 << SECTOR_SHIFT;
+		sg_set_page(&sg_out, bv_out->bv_page, 1 << SECTOR_SHIFT, bv_out->bv_offset + ctx->offset_out);
 
 		ctx->offset_in += sg_in.length;
 		if (ctx->offset_in >= bv_in->bv_len) {