MDEV-11520 post-fixes

Remove the unused variable desired_size.

Also, correct the expression for the posix_fallocate() start_offset,
and actually test that it works with a multi-file system tablespace.
Before MDEV-11520, the expression was wrong in both innodb_plugin and
xtradb, in different ways.

The start_offset formula was tested with the following:

./mtr --big-test --mysqld=--innodb-use-fallocate \
--mysqld=--innodb-data-file-path='ibdata1:5M;ibdata2:5M:autoextend' \
--parallel=auto --force --retry=0 --suite=innodb &

ls -lsh mysql-test/var/*/mysqld.1/data/ibdata2

storage/innobase/fil/fil0fil.c

@@ -4136,9 +4136,11 @@ fil_extend_space_to_desired_size(
 
 #ifdef HAVE_POSIX_FALLOCATE
 	if (srv_use_posix_fallocate) {
-		ib_int64_t start_offset = start_page_no * page_size;
-		ib_int64_t end_offset   = (size_after_extend - start_page_no) * page_size;
-		ib_int64_t desired_size = size_after_extend*page_size;
+
+		ib_int64_t	start_offset
+			= (start_page_no - file_start_page_no) * page_size;
+		ib_int64_t	end_offset
+			= (size_after_extend - file_start_page_no) * page_size;
 		int err = posix_fallocate(
 			node->handle, start_offset, end_offset);
 

storage/xtradb/fil/fil0fil.c

@@ -4973,7 +4973,7 @@ fil_extend_space_to_desired_size(
 	if (srv_use_posix_fallocate) {
 
 		ib_int64_t	start_offset
-			= file_start_page_no * page_size;
+			= (start_page_no - file_start_page_no) * page_size;
 		ib_int64_t	end_offset
 			= (size_after_extend - file_start_page_no) * page_size;
 		int err = posix_fallocate(