MDEV-6076: Preserve PAGE_ROOT_AUTO_INC when emptying pages.

Thanks to Zhangyuan from Alibaba for pointing out this bug.

btr_page_empty(): When a clustered index root page is emptied,
preserve PAGE_ROOT_AUTO_INC. This would occur during a page split.

page_create_empty(): Preserve PAGE_ROOT_AUTO_INC when a clustered
index root page becomes empty. Use a faster method for writing
the field.

page_zip_copy_recs(): Reset PAGE_MAX_TRX_ID when copying
clustered index pages. We must clear the field when the root page
was a leaf page and it is being split, so that PAGE_MAX_TRX_ID
will continue to be 0 in clustered index non-root pages.

page_create_zip(): Add debug assertions for validating
PAGE_MAX_TRX_ID and PAGE_ROOT_AUTO_INC.

mysql-test/suite/innodb/r/autoinc_persist.result

@@ -629,6 +629,16 @@ ERROR 0A000: LOCK=NONE is not supported. Reason: Adding an auto-increment column
 ALTER TABLE mdev6076a ADD COLUMN a SERIAL FIRST, ALGORITHM=INPLACE;
 ALTER TABLE mdev6076b ADD COLUMN a SERIAL FIRST, AUTO_INCREMENT=100,
 ALGORITHM=INPLACE;
+# MDEV-6076: Test root page split and page_create_empty()
+CREATE TABLE mdev6076empty (b SERIAL, pad CHAR(255) NOT NULL DEFAULT '')
+ENGINE=InnoDB;
+BEGIN;
+# Insert records in descending order of AUTO_INCREMENT,
+# causing a page split on the very last insert.
+# Without the fix in btr_page_empty() this would lose the counter value.
+# Without the fix in page_create_empty() the counter value would be lost
+# when ROLLBACK deletes the last row.
+ROLLBACK;
 # Kill and restart
 INSERT INTO t3 VALUES(0);
 SELECT MAX(a) AS `Expect 120` FROM t3;
@@ -646,7 +656,11 @@ a	b
 100	2
 101	1
 102	NULL
-DROP TABLE mdev6076a, mdev6076b;
+INSERT INTO mdev6076empty SET b=NULL;
+SELECT * FROM mdev6076empty;
+b	pad
+56	
+DROP TABLE mdev6076a, mdev6076b, mdev6076empty;
 INSERT INTO t3 VALUES(0), (0), (200), (210);
 # Test the different algorithms in ALTER TABLE
 CREATE TABLE t_inplace LIKE t3;