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