fil_extend_space_to_desired_size(): Use a proper type cast when
computing start_offset for the posix_fallocate() call on 32-bit systems
(where sizeof(ulint) < sizeof(os_offset_t)). This could affect 32-bit
systems when extending files that are at least 4 MiB long.

This bug existed in MariaDB 10.0 before MDEV-11520. In MariaDB 10.1
it had been fixed in MDEV-11556.