Merge branch 'ip6_gre-add-erspan-native-tunnel-for-ipv6'

William Tu says:

====================
ip6_gre: add erspan native tunnel for ipv6

The patch series add support for ERSPAN tunnel over ipv6.  The first patch
refectors the existing ipv4 gre implementation and the second refactors the
ipv6 gre's xmit code.  Finally the last patch introduces erspan protocol.

change in v5:
  - add cover-letter description

change in v4:
  - rebase on top of net-next
  - use log_ecn_error in ip6_tnl_rcv

change in v3:
  - add inline for functions in header
  - rebase on top of net-next

change in v2:
  - remove inline
  - fix some indent
  - fix errors reports by clang and scan-build
====================

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