net: fill in MODULE_DESCRIPTION()s under net/core

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().

Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>