migration/multifd: Print used_length of memory block

We actually want to print the used_length, against which we check.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210429112708.12291-10-david@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/multifd.c

@@ -361,7 +361,7 @@ static int multifd_recv_unfill_packet(MultiFDRecvParams *p, Error **errp)
         if (offset > (block->used_length - qemu_target_page_size())) {
             error_setg(errp, "multifd: offset too long %" PRIu64
                        " (max " RAM_ADDR_FMT ")",
-                       offset, block->max_length);
+                       offset, block->used_length);
             return -1;
         }
         p->pages->iov[i].iov_base = block->host + offset;