Merge branch 'ipv6-dgram-dst-cache'

Martin KaFai Lau says:

====================
ipv6: datagram: Update dst cache of a connected udp sk during pmtu update

v2:
~ Protect __sk_dst_get() operations with rcu_read_lock in
  release_cb() because another thread may do ip6_dst_store()
  for a udp sk without taking the sk lock (e.g. in sendmsg).
~ Do a ipv6_addr_v4mapped(&sk->sk_v6_daddr) check before
  calling ip6_datagram_dst_update() in patch 3 and 4.  It is
  similar to how __ip6_datagram_connect handles it.
~ One fix in ip6_datagram_dst_update() in patch 2.  It needs
  to check (np->flow_label & IPV6_FLOWLABEL_MASK) before
  doing fl6_sock_lookup.  I was confused with the naming
  of IPV6_FLOWLABEL_MASK and IPV6_FLOWINFO_MASK.
~ Check dst->obsolete just on the safe side, although I think it
  should at least have DST_OBSOLETE_FORCE_CHK by now.
~ Add Fixes tag to patch 3 and 4
~ Add some points from the previous discussion about holding
  sk lock to the commit message in patch 3.

v1:
There is a case in connected UDP socket such that
getsockopt(IPV6_MTU) will return a stale MTU value. The reproducible
sequence could be the following:
1. Create a connected UDP socket
2. Send some datagrams out
3. Receive a ICMPV6_PKT_TOOBIG
4. No new outgoing datagrams to trigger the sk_dst_check()
   logic to update the sk->sk_dst_cache.
5. getsockopt(IPV6_MTU) returns the mtu from the invalid
   sk->sk_dst_cache instead of the newly created RTF_CACHE clone.

Patch 1 and 2 are the prep work.
Patch 3 and 4 are the fixes.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>