journald: be more careful when we try to flush the runtime journal to disk and the disk is close to being full

Bump the minimal size of the journal so that we can be sure creating the
journal file will always succeed. Previously the minimum size was
smaller than a empty jounral file...

src/journal/journal-file.c

@@ -44,7 +44,7 @@
 #define COMPRESSION_SIZE_THRESHOLD (512ULL)
 
 /* This is the minimum journal file size */
-#define JOURNAL_FILE_SIZE_MIN (64ULL*1024ULL)                  /* 64 KiB */
+#define JOURNAL_FILE_SIZE_MIN (4ULL*1024ULL*1024ULL)           /* 4 MiB */
 
 /* These are the lower and upper bounds if we deduce the max_use value
  * from the file system size */

src/journal/journald-server.c

@@ -938,6 +938,12 @@ int server_flush_to_var(Server *s) {
                 server_rotate(s);
                 server_vacuum(s);
 
+                if (!s->system_journal) {
+                        log_notice("Didn't flush runtime journal since rotation of system journal wasn't successful.");
+                        r = -EIO;
+                        goto finish;
+                }
+
                 log_debug("Retrying write.");
                 r = journal_file_copy_entry(f, s->system_journal, o, f->current_offset, NULL, NULL, NULL);
                 if (r < 0) {