MDEV-18966 Transaction recovery may be broken after upgrade to 10.3

This bug was introduced by MDEV-12288, which made InnoDB use
a single undo log for persistent transactions, instead of
maintaining separate insert_undo and update_undo logs.

trx_undo_reuse_cached(): Initialize the TRX_UNDO_PAGE_TYPE
after reusing a cached undo log page for undo log.
Failure to do so can cause trx_undo_mem_create_at_db_start()
to misclassify new undo log records as TRX_UNDO_INSERT.
This in turn would trigger an assertion failure in
trx_roll_pop_top_rec_of_trx() due to undo==insert.

storage/innobase/trx/trx0undo.cc

@@ -1352,6 +1352,15 @@ trx_undo_reuse_cached(trx_t* trx, trx_rseg_t* rseg, trx_undo_t** pundo,
 	*pundo = undo;
 
 	ulint offset = trx_undo_header_create(block->frame, trx->id, mtr);
+	/* Reset the TRX_UNDO_PAGE_TYPE in case this page is being
+	repurposed after upgrading to MariaDB 10.3. */
+	if (ut_d(ulint type =) UNIV_UNLIKELY(
+		    mach_read_from_2(TRX_UNDO_PAGE_HDR + TRX_UNDO_PAGE_TYPE
+				     + block->frame))) {
+		ut_ad(type == TRX_UNDO_INSERT || type == TRX_UNDO_UPDATE);
+		mlog_write_ulint(TRX_UNDO_PAGE_HDR + TRX_UNDO_PAGE_TYPE
+				 + block->frame, 0, MLOG_2BYTES, mtr);
+	}
 
 	trx_undo_header_add_space_for_xid(block->frame, block->frame + offset,
 					  mtr);