block: add blk_rq_set_block_pc()

With the optimizations around not clearing the full request at alloc
time, we are leaving some of the needed init for REQ_TYPE_BLOCK_PC
up to the user allocating the request.

Add a blk_rq_set_block_pc() that sets the command type to
REQ_TYPE_BLOCK_PC, and properly initializes the members associated
with this type of request. Update callers to use this function instead
of manipulating rq->cmd_type directly.

Includes fixes from Christoph Hellwig <hch@lst.de> for my half-assed
attempt.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/bsg.c b/block/bsg.c
index e5214c1..ff46add 100644
--- a/block/bsg.c
+++ b/block/bsg.c
@@ -196,7 +196,6 @@
 	 * fill in request structure
 	 */
 	rq->cmd_len = hdr->request_len;
-	rq->cmd_type = REQ_TYPE_BLOCK_PC;
 
 	rq->timeout = msecs_to_jiffies(hdr->timeout);
 	if (!rq->timeout)
@@ -273,6 +272,8 @@
 	rq = blk_get_request(q, rw, GFP_KERNEL);
 	if (!rq)
 		return ERR_PTR(-ENOMEM);
+	blk_rq_set_block_pc(rq);
+
 	ret = blk_fill_sgv4_hdr_rq(q, rq, hdr, bd, has_write_perm);
 	if (ret)
 		goto out;