Increase default zfs_rebuild_vdev_limit to 64MB

When testing distributed rebuild performance with more capable
hardware it was observed than increasing the zfs_rebuild_vdev_limit
to 64M reduced the rebuild time by 17%.  Beyond 64MB there was
some improvement (~2%) but it was not significant when weighed
against the increased memory usage.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/vdev_rebuild.c

@@ -116,13 +116,12 @@ static uint64_t zfs_rebuild_max_segment = 1024 * 1024;
  * segment size is also large (zfs_rebuild_max_segment=1M).  This helps keep
  * the queue depth short.
  *
- * 32MB was selected as the default value to achieve good performance with
- * a large 90-drive dRAID HDD configuration (draid2:8d:90c:2s). A sequential
- * rebuild was unable to saturate all of the drives using smaller values.
- * With a value of 32MB the sequential resilver write rate was measured at
- * 800MB/s sustained while rebuilding to a distributed spare.
+ * 64MB was observed to deliver the best performance and set as the default.
+ * Testing was performed with a 106-drive dRAID HDD pool (draid2:11d:106c)
+ * and a rebuild rate of 1.2GB/s was measured to the distribute spare.
+ * Smaller values were unable to fully saturate the available pool I/O.
  */
-static uint64_t zfs_rebuild_vdev_limit = 32 << 20;
+static uint64_t zfs_rebuild_vdev_limit = 64 << 20;
 
 /*
  * Automatically start a pool scrub when the last active sequential resilver