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

@@ -4,7 +4,7 @@
  * Copyright (C) 2008 Timo Teräs <timo.teras@iki.fi>
  * All rights reserved.
  *
- * This program is free software; you can redistribute it and/or modify it 
+ * This program is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 as published
  * by the Free Software Foundation. See http://www.gnu.org/ for details.
  */
@@ -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);
 }