bpf: Add helper to do FIB lookups

Provide a helper for doing a FIB and neighbor lookup in the kernel tables from an XDP program. The helper provides a fastpath for forwarding packets. If the packet is a local delivery or for any reason is not a simple lookup and forward, the packet is expected to continue up the stack for full processing.

The response from a FIB and neighbor lookup is either the egress index with the bpf_fib_lookup struct filled in with dmac and gateway or 0 meaning the packet should continue up the stack. In time we can visit this to return the FIB lookup result errno if it is one of the special RTN_'s such as RTN_BLACKHOLE (-EINVAL) so that the XDP programs can do an early drop if desired.

Patches 1-6 do some more refactoring to IPv6 with the end goal of extracting a FIB lookup function that aligns with fib_lookup for IPv4, basically returning a fib6_info without creating a dst based entry.

Patch 7 adds lookup functions to the ipv6 stub. These are needed since bpf is built into the kernel and ipv6 may not be built or loaded.

Patch 8 adds the bpf helper and 9 adds a sample program.

v1
- updated commit messages and cover letter
- added comment to sample program noting lack of verification on egress device supporting XDP

RFC v2
- fixed use of foward helper from cls_act as noted by Daniel
- in patch 1 rename fib6_lookup_1 as well for consistency

David Ahern (9):
net/ipv6: Rename fib6_lookup to fib6_node_lookup net/ipv6: Rename rt6_multipath_select
net/ipv6: Extract table lookup from ip6_pol_route net/ipv6: Refactor fib6_rule_action
net/ipv6: Add fib6_lookup
net/ipv6: Update fib6 tracepoint to take fib6_info net/ipv6: Add fib lookup stubs for use in bpf helper
bpf: Provide helper to do lookups in kernel FIB table samples/bpf: Add example of ipv4 and ipv6 forwarding in XDP

 include/net/addrconf.h                    |  14 ++
 include/net/ip6_fib.h                     |  21 ++-
 include/trace/events/fib6.h               |  14 +-
 include/uapi/linux/bpf.h                  |  83 +++++++++-
 net/core/filter.c                         | 263 ++++++++++++++++++++++++++++++
 net/ipv6/addrconf_core.c                  |  33 +++-
 net/ipv6/af_inet6.c                       |   6 +-
 net/ipv6/fib6_rules.c                     | 138 +++++++++++++---
 net/ipv6/ip6_fib.c                        |  21 ++-
 net/ipv6/route.c                          |  76 +++++----
 samples/bpf/Makefile                      |   4 +
 samples/bpf/xdp_fwd_kern.c                | 113 +++++++++++++
 samples/bpf/xdp_fwd_user.c                | 136 +++++++++++++++
 tools/testing/selftests/bpf/bpf_helpers.h |   3 +
 14 files changed, 850 insertions(+), 75 deletions(-)
 create mode 100644 samples/bpf/xdp_fwd_kern.c
 create mode 100644 samples/bpf/xdp_fwd_user.c

2.11.0