ztest: creates partially initialized root dataset

Since d8fdfc2 was integrated dsl_pool_create() does not call
dmu_objset_create_impl() for the root dataset when running in
userland (ztest): this creates a pool with a partially initialized
root dataset. Trying to import and use this pool results in both
zpool and zfs executables dumping core.

Fix this by adopting an alternative change suggested in OpenZFS 8607
code review.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed by: Tom Caputi <[email protected]>
Original-patch-by: Robert Mustacchi <[email protected]>
Signed-off-by: loli10K <[email protected]>
Closes #8277

module/zfs/dsl_pool.c

@@ -457,6 +457,11 @@ dsl_pool_create(spa_t *spa, nvlist_t *zplprops, dsl_crypto_params_t *dcp,
 	int err;
 	dsl_pool_t *dp = dsl_pool_open_impl(spa, txg);
 	dmu_tx_t *tx = dmu_tx_create_assigned(dp, txg);
+#ifdef _KERNEL
+	objset_t *os;
+#else
+	objset_t *os __attribute__((unused));
+#endif
 	dsl_dataset_t *ds;
 	uint64_t obj;
 
@@ -520,15 +525,12 @@ dsl_pool_create(spa_t *spa, nvlist_t *zplprops, dsl_crypto_params_t *dcp,
 	/* create the root objset */
 	VERIFY0(dsl_dataset_hold_obj_flags(dp, obj,
 	    DS_HOLD_FLAG_DECRYPT, FTAG, &ds));
+	rrw_enter(&ds->ds_bp_rwlock, RW_READER, FTAG);
+	os = dmu_objset_create_impl(dp->dp_spa, ds,
+	    dsl_dataset_get_blkptr(ds), DMU_OST_ZFS, tx);
+	rrw_exit(&ds->ds_bp_rwlock, FTAG);
 #ifdef _KERNEL
-	{
-		objset_t *os;
-		rrw_enter(&ds->ds_bp_rwlock, RW_READER, FTAG);
-		os = dmu_objset_create_impl(dp->dp_spa, ds,
-		    dsl_dataset_get_blkptr(ds), DMU_OST_ZFS, tx);
-		rrw_exit(&ds->ds_bp_rwlock, FTAG);
-		zfs_create_fs(os, kcred, zplprops, tx);
-	}
+	zfs_create_fs(os, kcred, zplprops, tx);
 #endif
 	dsl_dataset_rele_flags(ds, DS_HOLD_FLAG_DECRYPT, FTAG);