Follow-up fix to MDEV-14952 for Mariabackup

innodb_init_param(): Initialize btr_ahi_parts=1 for Mariabackup.

btr_search_enabled: Let the adaptive hash index be disabled
in Mariabackup. This would potentially only matter during --export,
and --export performs a table scan, not many index lookups.

extra/mariabackup/xtrabackup.cc

@@ -1461,6 +1461,9 @@ innodb_init_param(void)
 
 	srv_page_size = 0;
 	srv_page_size_shift = 0;
+#ifdef BTR_CUR_HASH_ADAPT
+	btr_ahi_parts = 1;
+#endif /* BTR_CUR_HASH_ADAPT */
 
 	if (innobase_page_size != (1LL << 14)) {
 		int n_shift = (int)get_bit_shift((ulint) innobase_page_size);

storage/innobase/btr/btr0sea.cc

@@ -45,7 +45,7 @@ Created 2/17/1996 Heikki Tuuri
 
 /** Is search system enabled.
 Search system is protected by array of latches. */
-char		btr_search_enabled	= true;
+char		btr_search_enabled;
 
 /** Number of adaptive hash index partition. */
 ulong		btr_ahi_parts;