If the IPv6 fragment id has not been set and we perform
fragmentation due to UFO, select a new fragment id.
When we store the fragment id into skb_shinfo, set the bit
in the skb so we can re-use the selected id.
This preserves the behavior of UFO packets generated on the
host and solves the issue of id generation for packet sockets
and tap/macvtap devices.

This patch moves ipv6_select_ident() back in to the header file.  
It also provides the helper function that sets skb_shinfo() frag
id and sets the bit.

It also makes sure that we select the fragment id when doing
just gso validation, since it's possible for the packet to
come from an untrusted source (VM) and be forwarded through
a UFO enabled device which will expect the fragment id.

CC: Eric Dumazet <edumazet@google.com>
Signed-off-by: Vladislav Yasevich <redacted>
---
 include/linux/skbuff.h |  3 ++-
 include/net/ipv6.h     |  2 ++
 net/ipv6/ip6_output.c  |  4 ++--
 net/ipv6/output_core.c |  9 ++++++++-
 net/ipv6/udp_offload.c | 10 +++++++++-
 5 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 85ab7d7..3ad5203 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h

@@ -605,7 +605,8 @@ struct sk_buff {
 	__u8			ipvs_property:1;
 	__u8			inner_protocol_type:1;
 	__u8			remcsum_offload:1;
-	/* 3 or 5 bit hole */
+	__u8			ufo_fragid_set:1;