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.