MDEV-21259 Assertion failed in mtr_t::write()

btr_free_externally_stored_field(): Pass w=mtr_t::OPT to
note that the BTR_EXTERN_LEN is not necessarily changing
when a multi-page ROW_FORMAT=COMPRESSED off-page column
is being freed, and to allow redundant writes to the redo
log to be optimized away.

Ever since commit 56f6dab1d0
the refactored function mtr_t::write() asserts by default
that the page contents is being changed.

mysql-test/suite/innodb_zip/t/blob.test

@@ -0,0 +1,29 @@
+--source include/innodb_page_size_small.inc
+
+--echo #
+--echo # MDEV-21259 Assertion 'w != NORMAL || mach_read_from_4(ptr) != val'
+--echo # failed in mtr_t::write(), btr_free_externally_stored_field()
+--echo #
+
+SET @saved_frequency = @@GLOBAL.innodb_purge_rseg_truncate_frequency;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
+
+CREATE TABLE t1 (c TEXT, f2 INT PRIMARY KEY, f3 INT UNIQUE)
+ENGINE=InnoDB ROW_FORMAT=COMPRESSED KEY_BLOCK_SIZE=4;
+
+SET @level= @@GLOBAL.innodb_compression_level;
+SET GLOBAL innodb_compression_level=0;
+--connect (prevent_purge,localhost,root)
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+
+--connection default
+INSERT INTO t1 VALUES('a',1,0);
+UPDATE t1 SET c= REPEAT('a b ',4096);
+REPLACE INTO t1 SELECT * FROM t1;
+
+--disconnect prevent_purge
+--source ../../innodb/include/wait_all_purged.inc
+
+DROP TABLE t1;
+SET GLOBAL innodb_compression_level = @level;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;