Documentation: Use mail-archive.com URLs for openflow-discuss ML links.

The openflow-discuss mailing list archives at mailman.stanford.edu appear
to be offline. So use mail-archive.com URLs instead.

Flagged by make check-docs.

Reviewed-by: David Marchand <david.marchand@redhat.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Simon Horman <horms@ovn.org>

Author: Simon Horman

Documentation/topics/design.rst

@@ -459,7 +459,7 @@ is, the first byte of the encapsulated frame immediately follows the
 same way for compatibility.
 
 For an earlier discussion, please see the thread archived at:
-https://mailman.stanford.edu/pipermail/openflow-discuss/2011-August/002604.html
+https://www.mail-archive.com/openflow-discuss@lists.stanford.edu/msg00663.html
 
 [*] The quoted definition is directly from OF1.1.  Definitions used inside OVS
 omit the 8-byte ``ofp_header`` members, so the sizes in this discussion are

Documentation/topics/porting.rst

@@ -320,7 +320,7 @@ packet to software, slowing down not just UDPv6 but all IPv6 processing.
 
 .. [1] Aaron Rosen, "Modify packet fields extremely slow",
     openflow-discuss mailing list, June 26, 2011, archived at
-    https://mailman.stanford.edu/pipermail/openflow-discuss/2011-June/002386.html.
+    https://www.mail-archive.com/openflow-discuss@lists.stanford.edu/msg00447.html
 
 Questions
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