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.

*** 30,39 ****
--- 30,41 ----
  
  #include <sys/types.h>
  #include <sys/spa.h>
  #include <sys/zio.h>
  #include <sys/dmu.h>
+ #include <sys/dsl_pool.h>
+ #include <sys/dsl_dataset.h>
  
  #ifdef  __cplusplus
  extern "C" {
  #endif
  
*** 403,414 ****
  extern void     zil_itx_assign(zilog_t *zilog, itx_t *itx, dmu_tx_t *tx);
  
  extern void     zil_commit(zilog_t *zilog, uint64_t oid);
  
  extern int      zil_vdev_offline(const char *osname, void *txarg);
! extern int      zil_claim(const char *osname, void *txarg);
! extern int      zil_check_log_chain(const char *osname, void *txarg);
  extern void     zil_sync(zilog_t *zilog, dmu_tx_t *tx);
  extern void     zil_clean(zilog_t *zilog, uint64_t synced_txg);
  
  extern int      zil_suspend(const char *osname, void **cookiep);
  extern void     zil_resume(void *cookie);
--- 405,417 ----
  extern void     zil_itx_assign(zilog_t *zilog, itx_t *itx, dmu_tx_t *tx);
  
  extern void     zil_commit(zilog_t *zilog, uint64_t oid);
  
  extern int      zil_vdev_offline(const char *osname, void *txarg);
! extern int      zil_claim(dsl_pool_t *dp, dsl_dataset_t *ds, void *txarg);
! extern int      zil_check_log_chain(dsl_pool_t *dp, dsl_dataset_t *ds,
!     void *tx);
  extern void     zil_sync(zilog_t *zilog, dmu_tx_t *tx);
  extern void     zil_clean(zilog_t *zilog, uint64_t synced_txg);
  
  extern int      zil_suspend(const char *osname, void **cookiep);
  extern void     zil_resume(void *cookie);