mmap cache: invalidate cache on fatal error
    
    If a fatal EFAULT error is returned by the internal function that frees used
    memory invalidate the whole cache and reinit it. This way we avoid further
    corruption and insure clients see consistent data.
    
    Also insure we use the right context in init() and we use talloc_zfree() in
    reinit so that if the init() later fails we do not leave around a pointer
    to free memory in the callers.