The 8 bytes at FIL_PAGE_FILE_FLUSH_LSN_OR_KEY_VERSION
will be overwritten at page flush, in buf_page_encrypt_before_write(),
ever since commit 765a4360
(MariaDB 10.1.23, 10.2.6, 10.3.1).

btr_store_big_rec_extern_fields(): Remove useless writes to the
page header (and to the redo log) for ROW_FORMAT=COMPRESSED BLOB pages.