MDEV-12075 innodb_use_fallocate does not work in MariaDB Server 10.1.21

fil_space_extend_must_retry(): When innodb_use_fallocate=ON,
initialize pages_added = size - space->size so that posix_fallocate()
will actually attempt to extend the file, instead of keeping the same size.

This is a regression from MDEV-11556 which refactored
the InnoDB data file extension.

storage/innobase/fil/fil0fil.cc

@@ -1054,6 +1054,8 @@ fil_space_extend_must_retry(
 		? OS_FILE_READ : OS_FILE_WRITE;
 
 	if (srv_use_posix_fallocate) {
+		pages_added = size - space->size;
+
 		const os_offset_t start_offset	= static_cast<os_offset_t>(
 			start_page_no) * page_size;
 		const os_offset_t len		= static_cast<os_offset_t>(

storage/xtradb/fil/fil0fil.cc

@@ -1059,6 +1059,8 @@ fil_space_extend_must_retry(
 		? OS_FILE_READ : OS_FILE_WRITE;
 
 	if (srv_use_posix_fallocate) {
+		pages_added = size - space->size;
+
 		const os_offset_t start_offset	= static_cast<os_offset_t>(
 			start_page_no) * page_size;
 		const os_offset_t len		= static_cast<os_offset_t>(