zpool import speedup

*** 568,577 ****
--- 568,578 ----
          SPA_IMPORT_ASSEMBLE
  } spa_import_type_t;
  
  /* state manipulation functions */
  extern int spa_open(const char *pool, spa_t **, void *tag);
+ extern int spa_open_lock(const char *pool, spa_t **, void *tag, int lock);
  extern int spa_open_rewind(const char *pool, spa_t **, void *tag,
      nvlist_t *policy, nvlist_t **config);
  extern int spa_get_stats(const char *pool, nvlist_t **config, char *altroot,
      size_t buflen);
  extern int spa_create(const char *pool, nvlist_t *config, nvlist_t *props,