Apply a few spa_writeable() changes that were made to Illumos in

illumos/illumos-gate.git@cd1c8b8 as part of the 3112, 3113,
3114 and 3115 fixes.

Add a missing call to fnvlist_free(nvl) in log_internal() that
was added in Illumos to fix issue 3085 but couldn't be ported
to ZoL at the time (openzfs/zfs@9e11c73) because it depended
on future work.

module/zfs/dsl_synctask.c

@@ -234,9 +234,6 @@ dsl_sync_task_do_nowait(dsl_pool_t *dp,
 {
 	dsl_sync_task_group_t *dstg;
 
-	if (!spa_writeable(dp->dp_spa))
-		return;
-
 	dstg = dsl_sync_task_group_create(dp);
 	dsl_sync_task_create(dstg, checkfunc, syncfunc,
 	    arg1, arg2, blocks_modified);

module/zfs/spa_history.c

@@ -305,7 +305,7 @@ spa_history_log_nvl(spa_t *spa, nvlist_t *nvl)
 	dmu_tx_t *tx;
 	nvlist_t *nvarg;
 
-	if (spa_version(spa) < SPA_VERSION_ZPOOL_HISTORY)
+	if (spa_version(spa) < SPA_VERSION_ZPOOL_HISTORY || !spa_writeable(spa))
 		return (EINVAL);
 
 	tx = dmu_tx_create_dd(spa_get_dsl(spa)->dp_mos_dir);
@@ -442,9 +442,12 @@ log_internal(nvlist_t *nvl, const char *operation, spa_t *spa,
 	/*
 	 * If this is part of creating a pool, not everything is
 	 * initialized yet, so don't bother logging the internal events.
+	 * Likewise if the pool is not writeable.
 	 */
-	if (tx->tx_txg == TXG_INITIAL)
+	if (tx->tx_txg == TXG_INITIAL || !spa_writeable(spa)) {
+		fnvlist_free(nvl);
 		return;
+	}
 
 	va_copy(adx1, adx);
 	msg = kmem_alloc(vsnprintf(NULL, 0, fmt, adx1) + 1, KM_PUSHPAGE);