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.

@@ -135,10 +135,12 @@
 
 /* called from zpl */
 int dmu_objset_hold(const char *name, void *tag, objset_t **osp);
 int dmu_objset_own(const char *name, dmu_objset_type_t type,
     boolean_t readonly, void *tag, objset_t **osp);
+int dmu_objset_own_obj(dsl_pool_t *dp, uint64_t obj, dmu_objset_type_t type,
+    boolean_t readonly, void *tag, objset_t **osp);
 void dmu_objset_refresh_ownership(objset_t *os, void *tag);
 void dmu_objset_rele(objset_t *os, void *tag);
 void dmu_objset_disown(objset_t *os, void *tag);
 int dmu_objset_from_ds(struct dsl_dataset *ds, objset_t **osp);