scsi: Implement blk_mq_ops.show_rq()

Show the SCSI CDB for pending SCSI commands in
/sys/kernel/debug/block/*/mq/*/dispatch and */rq_list. An example
of how SCSI commands are displayed by this code:

ffff8801703245c0 {.op=READ, .cmd_flags=META PRIO, .rq_flags=DONTPREP IO_STAT STATS, .tag=14, .internal_tag=-1, .cmd=Read(10) 28 00 2a 81 1b 30 00 00 08 00}

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: <linux-scsi@vger.kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/scsi/scsi_debugfs.c b/drivers/scsi/scsi_debugfs.c
new file mode 100644
index 0000000..a97c950
--- /dev/null
+++ b/drivers/scsi/scsi_debugfs.c
@@ -0,0 +1,13 @@
+#include <linux/seq_file.h>
+#include <scsi/scsi_cmnd.h>
+#include <scsi/scsi_dbg.h>
+#include "scsi_debugfs.h"
+
+void scsi_show_rq(struct seq_file *m, struct request *rq)
+{
+	struct scsi_cmnd *cmd = container_of(scsi_req(rq), typeof(*cmd), req);
+	char buf[80];
+
+	__scsi_format_command(buf, sizeof(buf), cmd->cmnd, cmd->cmd_len);
+	seq_printf(m, ", .cmd=%s", buf);
+}