ide-taskfile: don't abuse rq->buffer

Impact: rq->buffer usage cleanup

ide_raw_taskfile() directly uses rq->buffer to carry pointer to the
data buffer.  This complicates both block interface and ide backend
request handling.  Use blk_rq_map_kern() instead and drop special
handling for REQ_TYPE_ATA_TASKFILE from ide_map_sg().

Note that REQ_RW setting is moved upwards as blk_rq_map_kern() uses it
to initialize bio rw flag.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
diff --git a/drivers/ide/ide-io.c b/drivers/ide/ide-io.c
index 35dc38d..9b9e8b1 100644
--- a/drivers/ide/ide-io.c
+++ b/drivers/ide/ide-io.c
@@ -248,10 +248,7 @@
 	struct scatterlist *sg = hwif->sg_table;
 	struct request *rq = cmd->rq;
 
-	if (rq->cmd_type == REQ_TYPE_ATA_TASKFILE) {
-		sg_init_one(sg, rq->buffer, rq->nr_sectors * SECTOR_SIZE);
-		cmd->sg_nents = 1;
-	} else if (!rq->bio) {
+	if (!rq->bio) {
 		sg_init_one(sg, rq->data, rq->data_len);
 		cmd->sg_nents = 1;
 	} else