io: fix mmap bstream fd leak

We need to close the fd on destruction. This is what the corresponding
istream variant does too.

src/io.c

@@ -257,6 +257,7 @@ static void mmap_close(void *stream, csum_t csum, size_t *size)
 		*size = mbs->size;
 
 	munmap(mbs->ptr, mbs->size);
+	close(mbs->fd);
 	free(mbs);
 }