esp6: calculate transport_header correctly when sel.family != AF_INET6

In esp6_init_state() for beet mode when x->sel.family != AF_INET6:

  x->props.header_len = sizeof(struct ip_esp_hdr) +
     crypto_aead_ivsize(aead) + IPV4_BEET_PHMAXLEN +
     (sizeof(struct ipv6hdr) - sizeof(struct iphdr))

In xfrm6_beet_gso_segment() skb->transport_header is supposed to move
to the end of the ph header for IPPROTO_BEETPH, so if x->sel.family !=
AF_INET6 and it's IPPROTO_BEETPH, it should do:

   skb->transport_header -=
      (sizeof(struct ipv6hdr) - sizeof(struct iphdr));
   skb->transport_header += ph->hdrlen * 8;

And IPV4_BEET_PHMAXLEN is only reserved for PH header, so if
x->sel.family != AF_INET6 and it's not IPPROTO_BEETPH, it should do:

   skb->transport_header -=
      (sizeof(struct ipv6hdr) - sizeof(struct iphdr));
   skb->transport_header -= IPV4_BEET_PHMAXLEN;

Thanks Sabrina for looking deep into this issue.

Fixes: 7f9e40eb

 ("esp6: add gso_segment for esp6 beet mode")
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>