ipv6: xfrm: use full sockets in local error paths

xfrm6_local_rxpmtu() and xfrm6_local_error() dereference skb->sk as if it
always pointed at a full IPv6 socket.

That is not guaranteed. TCP SYN-ACK skbs can be owned by a
TCP_NEW_SYN_RECV request_sock while the output path itself is driven by the
full listener. If rerouting selects an IPv6 XFRM tunnel route with a lower
MTU, the local PMTU/error handling path can reach these callbacks with that
mini-socket still attached to the skb.

The callbacks then miscast the request socket as a full inet/IPv6 socket and
can read beyond the request_sock allocation when they access inet_sock or
ipv6_pinfo state.

Resolve the owner with skb_to_full_sk() in both callbacks and bail out when
no full socket is attached. This matches the surrounding XFRM IPv6 PMTU/error
logic, which already reasons about full sockets with skb_to_full_sk().

Fixes: dd767856a36e ("xfrm6: Don't call icmpv6_send on local error")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>