Fixed unneeded memcpy on it self in zio_ready(zio_t *zio).

In zio_ready(), under some condition a blockpointer is copied into
itself, i.e. bptr_t bp1, *bp2;  bp2 = &bp;  bp1 = *bp2;
This produces a memcpy overlap error in valgrind.

The real question of course is, if trying to copy the blockpointer
into itself is a legitimate operation here.  Not sure.

Call tree for the offending copy:
==00:00:00:02.705 27867== Source and destination overlap in memcpy(0x1004b94f8, 0x1004b94f8, 128)
==00:00:00:02.705 27867==    at 0x10016AD73: memcpy (mc_replace_strmem.c:523)
==00:00:00:02.705 27867==    by 0x1000B954C: zio_ready (zio.c:919)
==00:00:00:02.705 27867==    by 0x1000BE103: zio_next_stage (zio.c:2024)
==00:00:00:02.705 27867==    by 0x1000B9192: zio_wait_for_children (zio.c:846)
==00:00:00:02.705 27867==    by 0x1000B93BA: zio_wait_children_ready (zio.c:874)
==00:00:00:02.705 27867==    by 0x1000BE38A: zio_next_stage_async (zio.c:2073)
==00:00:00:02.705 27867==    by 0x1000B9111: zio_nowait (zio.c:833)
==00:00:00:02.705 27867==    by 0x10009B7D8: vdev_label_read (vdev_label.c:168)
==00:00:00:02.705 27867==    by 0x10009C69D: vdev_label_read_config (vdev_label.c:336)
==00:00:00:02.705 27867==    by 0x100096FD3: vdev_validate (vdev.c:1009)
==00:00:00:02.705 27867==    by 0x100096F80: vdev_validate (vdev.c:999)
==00:00:00:02.705 27867==    by 0x100096F80: vdev_validate (vdev.c:999)
==00:00:00:02.705 27867==

usr/src/uts/common/fs/zfs/zio.c

@@ -916,7 +916,8 @@ zio_ready(zio_t *zio)
 		    &pio->io_children_notready);
 
 	if (zio->io_bp)
-		zio->io_bp_copy = *zio->io_bp;
+		if (&zio->io_bp_copy != zio->io_bp) /* do not copy block over itself. */
+			zio->io_bp_copy = *zio->io_bp;
 
 	zio_next_stage(zio);
 }