ipv6: Count in extension headers in skb->network_header

When sending a UDPv6 message longer than MTU, account for the length
of fragmentable IPv6 extension headers in skb->network_header offset.
Same as we do in alloc_new_skb path in __ip6_append_data().

This ensures that later on __ip6_make_skb() will make space in
headroom for fragmentable extension headers:

	/* move skb->data to ip header from ext header */
	if (skb->data < skb_network_header(skb))
		__skb_pull(skb, skb_network_offset(skb));

Prevents a splat due to skb_under_panic:

skbuff: skb_under_panic: text:ffffffff8143397b len:2126 put:14 \
head:ffff880005bacf50 data:ffff880005bacf4a tail:0x48 end:0xc0 dev:lo
------------[ cut here ]------------
kernel BUG at net/core/skbuff.c:104!
invalid opcode: 0000 [#1] KASAN
CPU: 0 PID: 160 Comm: reproducer Not tainted 4.6.0-rc2 #65
[...]
Call Trace:
 [<ffffffff813eb7b9>] skb_push+0x79/0x80
 [<ffffffff8143397b>] eth_header+0x2b/0x100
 [<ffffffff8141e0d0>] neigh_resolve_output+0x210/0x310
 [<ffffffff814eab77>] ip6_finish_output2+0x4a7/0x7c0
 [<ffffffff814efe3a>] ip6_output+0x16a/0x280
 [<ffffffff815440c1>] ip6_local_out+0xb1/0xf0
 [<ffffffff814f1115>] ip6_send_skb+0x45/0xd0
 [<ffffffff81518836>] udp_v6_send_skb+0x246/0x5d0
 [<ffffffff8151985e>] udpv6_sendmsg+0xa6e/0x1090
[...]

Reported-by: Ji Jianwen <jiji@redhat.com>
Signed-off-by: Jakub Sitnicki <jkbs@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>