5269 zfs: zpool import slow
While importing a pool all objsets are enumerated twice, once to check
the zil log chains and once to claim them. On pools with many datasets
this process might take a substantial amount of time.
Speed up the process by parallelizing it utilizing a taskq. The number
of parallel tasks is limited to 4 times the number of leaf vdevs.

*** 170,179 ****
--- 170,200 ----
                          return (mvd);
  
          return (NULL);
  }
  
+ static int
+ vdev_count_leaves_impl(vdev_t *vd)
+ {
+         vdev_t *mvd;
+         int n = 0;
+ 
+         if (vd->vdev_children == 0)
+                 return (1);
+ 
+         for (int c = 0; c < vd->vdev_children; c++)
+                 n += vdev_count_leaves_impl(vd->vdev_child[c]);
+ 
+         return (n);
+ }
+ 
+ int
+ vdev_count_leaves(spa_t *spa)
+ {
+         return (vdev_count_leaves_impl(spa->spa_root_vdev));
+ }
+ 
  void
  vdev_add_child(vdev_t *pvd, vdev_t *cvd)
  {
          size_t oldsize, newsize;
          uint64_t id = cvd->vdev_id;