Illumos 4168, 4169, 4170: ztest, zdb and zhack fixes

4168 ztest assertion failure in dbuf_undirty
4169 verbatim import causes zdb to segfault
4170 zhack leaves pool in ACTIVE state
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:
    https://www.illumos.org/issues/4168
    https://www.illumos.org/issues/4169
    https://www.illumos.org/issues/4170
    https://github.com/illumos/illumos-gate/commit/7fdd916

Porting notes:

Of particular interest when troubleshooting corrupted pools, the
commonly-used "zdb -e" operation may perform verbatim imports and
furthermore, it will soon have direct support for verbatim imports via
a new "-V" option.  The 4169 fix eliminates a common segfault case in
which spa_history_log_version() tries to access an un-opened dsl_pool_t.

Ported by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2451
Closes #2283
Closes #2467

module/zfs/dbuf.c

@@ -1391,14 +1391,6 @@ dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
 	DB_DNODE_ENTER(db);
 	dn = DB_DNODE(db);
 
-	/*
-	 * Note:  This code will probably work even if there are concurrent
-	 * holders, but it is untested in that scenerio, as the ZPL and
-	 * ztest have additional locking (the range locks) that prevents
-	 * that type of concurrent access.
-	 */
-	ASSERT3U(refcount_count(&db->db_holds), ==, db->db_dirtycnt);
-
 	dprintf_dbuf(db, "size=%llx\n", (u_longlong_t)db->db.db_size);
 
 	ASSERT(db->db.db_size != 0);

module/zfs/spa.c

@@ -3901,8 +3901,6 @@ spa_import(char *pool, nvlist_t *config, nvlist_t *props, uint64_t flags)
 		spa_config_sync(spa, B_FALSE, B_TRUE);
 
 		mutex_exit(&spa_namespace_lock);
-		spa_history_log_version(spa, "import");
-
 		return (0);
 	}