xfrm interface: fix packet tx through bpf_redirect()

xfrm interface: fix packet tx through bpf_redirect()

With an ebpf program that redirects packets through a xfrm interface,
packets are dropped because no dst is attached to skb.

This could also be reproduced with an AF_PACKET socket, with the following
python script (xfrm1 is a xfrm interface):

 import socket
 send_s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, 0)
 # scapy
 # p = IP(src='10.100.0.2', dst='10.200.0.1')/ICMP(type='echo-request')
 # raw(p)
 req = b'E\x00\x00\x1c\x00\x01\x00\x00@\x01e\xb2\nd\x00\x02\n\xc8\x00\x01\x08\x00\xf7\xff\x00\x00\x00\x00'
 send_s.sendto(req, ('xfrm1', 0x800, 0, 0))

It was also not possible to send an ip packet through an AF_PACKET socket
because a LL header was expected. Let's remove those LL header constraints.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>