nvme/scsi: don't rely on BLK_MAX_CDB

The NVMe SCSI emulation doesn't use BLOCK_PC requests, so BLK_MAX_CDB
doesn't have a meaning for it.  Instead opencode the value of 16
and refactor the code a bit so that related checks are next to each
other and we only need to use the value in one place.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/nvme/host/scsi.c

@@ -2348,12 +2348,14 @@ static int nvme_trans_unmap(struct nvme_ns *ns, struct sg_io_hdr *hdr,
 
 static int nvme_scsi_translate(struct nvme_ns *ns, struct sg_io_hdr *hdr)
 {
-	u8 cmd[BLK_MAX_CDB];
+	u8 cmd[16];
 	int retcode;
 	unsigned int opcode;
 
 	if (hdr->cmdp == NULL)
 		return -EMSGSIZE;
+	if (hdr->cmd_len > sizeof(cmd))
+		return -EINVAL;
 	if (copy_from_user(cmd, hdr->cmdp, hdr->cmd_len))
 		return -EFAULT;
 
@@ -2452,8 +2454,6 @@ int nvme_sg_io(struct nvme_ns *ns, struct sg_io_hdr __user *u_hdr)
 		return -EFAULT;
 	if (hdr.interface_id != 'S')
 		return -EINVAL;
-	if (hdr.cmd_len > BLK_MAX_CDB)
-		return -EINVAL;
 
 	/*
 	 * A positive return code means a NVMe status, which has been