Fix mlockall()/madvise() interaction.

mlockall(2) can cause purging via madvise(2) to fail.  Fix purging code
to check whether madvise() succeeded, and base zeroed page metadata on
the result.

Reported by Olivier Lecomte.

include/jemalloc/internal/extent.h

@@ -23,6 +23,9 @@ struct extent_node_s {
 
 	/* Total region size. */
 	size_t			size;
+
+	/* True if zero-filled; used by chunk recycling code. */
+	bool			zeroed;
 };
 typedef rb_tree(extent_node_t) extent_tree_t;