pfcp: avoid copy warning by simplifing code

From Arnd comments:
"The memcpy() in the ip_tunnel_info_opts_set() causes
a string.h fortification warning, with at least gcc-13:

    In function 'fortify_memcpy_chk',
        inlined from 'ip_tunnel_info_opts_set' at include/net/ip_tunnels.h:619:3,
        inlined from 'pfcp_encap_recv' at drivers/net/pfcp.c:84:2:
    include/linux/fortify-string.h:553:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
      553 |                         __write_overflow_field(p_size_field, size);"

It is a false-positivie caused by ambiguity of the union.

However, as Arnd noticed, copying here is unescessary. The code can be
simplified to avoid calling ip_tunnel_info_opts_set(), which is doing
copying, setting flags and options_len.

Set correct flags and options_len directly on tun_info.

Fixes: 6dd514f4 ("pfcp: always set pfcp metadata")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/netdev/701f8f93-f5fb-408b-822a-37a1d5c424ba@app.fastmail.com/Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pfcp.c

@@ -80,9 +80,8 @@ static int pfcp_encap_recv(struct sock *sk, struct sk_buff *skb)
 	else
 		pfcp_node_recv(pfcp, skb, md);
 
-	__set_bit(IP_TUNNEL_PFCP_OPT_BIT, flags);
-	ip_tunnel_info_opts_set(&tun_dst->u.tun_info, md, sizeof(*md),
-				flags);
+	__set_bit(IP_TUNNEL_PFCP_OPT_BIT, tun_dst->u.tun_info.key.tun_flags);
+	tun_dst->u.tun_info.options_len = sizeof(*md);
 
 	if (unlikely(iptunnel_pull_header(skb, PFCP_HLEN, skb->protocol,
 					  !net_eq(sock_net(sk),