MDEV-18589 Assertion on info.page_size failed in xb_delta_open_matching_space

xb_read_delta_metadata(): For ROW_FORMAT=COMPRESSED tables, initialize
the info.zip_size with the physical page size and let info.page_size
remain the logical page size, like xb_delta_open_matching_space()
expects it to be ever since
commit 0a1c3477 (MDEV-18493).

extra/mariabackup/xtrabackup.cc

@@ -2411,7 +2411,8 @@ xb_read_delta_metadata(const char *filepath, xb_delta_info_t *info)
 		msg("page_size is required in %s", filepath);
 		r = FALSE;
 	} else {
-		info->page_size = zip_size ? zip_size : page_size;
+		info->page_size = page_size;
+		info->zip_size = zip_size;
 	}
 
 	if (info->space_id == ULINT_UNDEFINED) {

mysql-test/suite/mariabackup/incremental_compressed.result

+#
+# MDEV-18589 Assertion ...physical_size(flags) == info.page_size
+#            failed in xb_delta_open_matching_space
+#
+CREATE TABLE t (pk INT PRIMARY KEY) ENGINE=InnoDB ROW_FORMAT=COMPRESSED;
+ALTER TABLE t PARTITION BY KEY(pk);
+# shutdown server
+# remove datadir
+# xtrabackup move back
+# restart
+SHOW CREATE TABLE t;
+Table	Create Table
+t	CREATE TABLE `t` (
+  `pk` int(11) NOT NULL,
+  PRIMARY KEY (`pk`)
+) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci ROW_FORMAT=COMPRESSED
+ PARTITION BY KEY (`pk`)
+DROP TABLE t;

mysql-test/suite/mariabackup/incremental_compressed.test

+--source include/have_innodb.inc
+--source include/have_partition.inc
+
+let $basedir=$MYSQLTEST_VARDIR/tmp/backup;
+let $incremental_dir=$MYSQLTEST_VARDIR/tmp/backup_inc1;
+
+--echo #
+--echo # MDEV-18589 Assertion ...physical_size(flags) == info.page_size
+--echo #            failed in xb_delta_open_matching_space
+--echo #
+
+CREATE TABLE t (pk INT PRIMARY KEY) ENGINE=InnoDB ROW_FORMAT=COMPRESSED;
+
+--exec $XTRABACKUP --backup --target-dir=$basedir --protocol=tcp --port=$MASTER_MYPORT --user=root
+
+ALTER TABLE t PARTITION BY KEY(pk);
+
+--exec $XTRABACKUP --backup --target-dir=$incremental_dir --incremental-basedir=$basedir --protocol=tcp --port=$MASTER_MYPORT --user=root > $incremental_dir.log 2>&1
+--exec $XTRABACKUP --prepare --target-dir=$basedir --user=root > $MYSQL_TMP_DIR/backup_prepare_0.log 2>&1
+--exec $XTRABACKUP --prepare --target-dir=$basedir --incremental-dir=$incremental_dir --user=root >  $MYSQL_TMP_DIR/backup_prepare_1.log
+--cat_file $MYSQL_TMP_DIR/backup_prepare_1.log
+let $targetdir=$basedir;
+-- source include/restart_and_restore.inc
+
+SHOW CREATE TABLE t;
+DROP TABLE t;