gtp: support for IPv4-in-IPv6-GTP and IPv6-in-IPv4-GTP

Add new protocol field to PDP context that determines the transmit path
IP protocol to encapsulate the original packets, either IPv4 or IPv6.

Relax existing netlink attribute checks to allow to specify different
family in MS and peer attributes from the control plane.

Use build helpers to tx path to encapsulate IPv4-in-IPv6-GTP and
IPv6-in-IPv4-GTP according to the user-specified configuration.

From rx path, snoop for the inner protocol header since outer
skb->protocol might differ and use this to validate for valid PDP
context and to restore skb->protocol after decapsulation.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>