MAINTAINERS: add David Ahern to IPv4/IPv6 maintainers

David has been the de-facto maintainer for much of the IP code
for the last couple of years, let's make it official.

Link: https://lore.kernel.org/r/20210122173220.3579491-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>