Documentation: Fix references to defunct linux-net@vger.kernel.org

linux-net@vger.kernel.org was replaced by netdev@oss.sgi.com was replaced
by netdev@vger.kernel.org.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>