MDEV-21405 Assertion failed on instant ADD COLUMN

btr_cur_pessimistic_insert(): Relax a too strict debug assertion that
would fail when the function is invoked by btr_cur_pessimistic_update()
during innobase_add_instant_try(), that is, when updating the hidden
metadata record during a subsequent ADD COLUMN operation involves
splitting the leftmost clustered index leaf page.

This is a partial backport of 301bd62b
from 10.4.

storage/innobase/btr/btr0cur.cc

@@ -3666,8 +3666,8 @@ btr_cur_pessimistic_insert(
 		if (entry->info_bits & REC_INFO_MIN_REC_FLAG) {
 			ut_ad(entry->info_bits == REC_INFO_METADATA);
 			ut_ad(index->is_instant());
-			ut_ad((flags & ulint(~BTR_KEEP_IBUF_BITMAP))
-			      == BTR_NO_LOCKING_FLAG);
+			ut_ad(flags & BTR_NO_LOCKING_FLAG);
+			ut_ad(!(flags & BTR_CREATE_FLAG));
 		} else {
 			btr_search_update_hash_on_insert(
 				cursor, btr_get_search_latch(index));