MDEV-24670 memory pressure - mariadb-backup postfix

mariadb-backup wasn't meant to have memory pressure
sensors so restrict the operation to SRV_OPERATIONAL_NORMAL
mode.

Reviewed by Daniel Black.

storage/innobase/buf/buf0buf.cc

@@ -1309,6 +1309,7 @@ bool buf_pool_t::create()
   btr_search_sys_create();
 
 #ifdef __linux__
+  if (srv_operation == SRV_OPERATION_NORMAL)
     buf_mem_pressure_detect_init();
 #endif
   ut_ad(is_initialised());