journal: fix hashmap leak in mmap-cache

journal: fix hashmap leak in mmap-cache

hashmap_free() wasn't being called on m->contexts and m->fds resulting
in a leak.

To reproduce do:
	while(1) {
		sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
		sd_journal_close(j);
	}

Memory usage will increase until OOM.