4171 clean up spa_feature_*() interfaces
4172 implement extensible_dataset feature for use by other zpool features
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

*** 570,580 ****
          kmem_free(mzp, sz);
          *zapp = zap;
          return (err);
  }
  
! static void
  mzap_create_impl(objset_t *os, uint64_t obj, int normflags, zap_flags_t flags,
      dmu_tx_t *tx)
  {
          dmu_buf_t *db;
          mzap_phys_t *zp;
--- 570,580 ----
          kmem_free(mzp, sz);
          *zapp = zap;
          return (err);
  }
  
! void
  mzap_create_impl(objset_t *os, uint64_t obj, int normflags, zap_flags_t flags,
      dmu_tx_t *tx)
  {
          dmu_buf_t *db;
          mzap_phys_t *zp;
*** 860,871 ****
  }
  
  int
  zap_contains(objset_t *os, uint64_t zapobj, const char *name)
  {
!         int err = (zap_lookup_norm(os, zapobj, name, 0,
!             0, NULL, MT_EXACT, NULL, 0, NULL));
          if (err == EOVERFLOW || err == EINVAL)
                  err = 0; /* found, but skipped reading the value */
          return (err);
  }
  
--- 860,871 ----
  }
  
  int
  zap_contains(objset_t *os, uint64_t zapobj, const char *name)
  {
!         int err = zap_lookup_norm(os, zapobj, name, 0,
!             0, NULL, MT_EXACT, NULL, 0, NULL);
          if (err == EOVERFLOW || err == EINVAL)
                  err = 0; /* found, but skipped reading the value */
          return (err);
  }