bsg: fix bogus EINVAL on non-data commands

Fix a regression introduced in Linux kernel 4.17 where sending a SCSI
command that does not transfer data (such as TEST UNIT READY) via
/dev/bsg/* results in EINVAL.

Fixes: 17cb960f29c2 ("bsg: split handling of SCSI CDBs vs transport requeues")
Cc: <stable@vger.kernel.org> # 4.17+
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bsg.c

@@ -267,8 +267,6 @@ bsg_map_hdr(struct request_queue *q, struct sg_io_v4 *hdr, fmode_t mode)
 	} else if (hdr->din_xfer_len) {
 		ret = blk_rq_map_user(q, rq, NULL, uptr64(hdr->din_xferp),
 				hdr->din_xfer_len, GFP_KERNEL);
-	} else {
-		ret = blk_rq_map_user(q, rq, NULL, NULL, 0, GFP_KERNEL);
 	}
 
 	if (ret)