netdev-FAQ: clarify DaveM's position for stable backports

Per discussion with David at netconf 2018, let's clarify
DaveM's position of handling stable backports in netdev-FAQ.

This is important for people relying on upstream -stable
releases.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/netdev-FAQ.txt

@@ -179,6 +179,15 @@ A: No.  See above answer.  In short, if you think it really belongs in
    dash marker line as described in Documentation/process/submitting-patches.rst to
    temporarily embed that information into the patch that you send.
 
+Q: Are all networking bug fixes backported to all stable releases?
+
+A: Due to capacity, Dave could only take care of the backports for the last
+   2 stable releases. For earlier stable releases, each stable branch maintainer
+   is supposed to take care of them. If you find any patch is missing from an
+   earlier stable branch, please notify stable@vger.kernel.org with either a
+   commit ID or a formal patch backported, and CC Dave and other relevant
+   networking developers.
+
 Q: Someone said that the comment style and coding convention is different
    for the networking content.  Is this true?