net: Convert CONFIG_IP_DEFRAG to Kconfig

Convert CONFIG_IP_DEFRAG to Kconfig, update defconfigs, headers
and whitelist. This patch is a follow-up on a patch by Christian
Gmeiner with the added config/header/whitelist updates.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Reported-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>