Skip spurious resilver IO on raidz vdev

On a raidz vdev, a block that does not span all child vdevs, excluding
its skip sectors if any, may not be affected by a child vdev outage or
failure. In such cases, the block does not need to be resilvered.
However, current resilver algorithm simply resilvers all blocks on a
degraded raidz vdev. Such spurious IO is not only wasteful, but also
adds the risk of overwriting good data.

This patch eliminates such spurious IOs.

Reviewed-by: Gvozden Neskovic <neskovic@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Isaac Huang <he.huang@intel.com>
Closes #5316

module/zfs/vdev_root.c

@@ -120,6 +120,7 @@ vdev_ops_t vdev_root_ops = {
 	vdev_root_state_change,
 	NULL,
 	NULL,
+	NULL,
 	VDEV_TYPE_ROOT,		/* name of this vdev type */
 	B_FALSE			/* not a leaf vdev */
 };