Fully allocate file backing writable maps (#389)

Fully allocate file backing writable maps (#389)

When ftruncate() is used on a filesystem supporting sparse files,
space in the file is not actually allocated.  Then, when the file
is mmap'd and libffi writes to the mapping, SIGBUS is thrown to
the calling application.  Instead, always fully allocate the file
that will back writable maps.