Fix the handling of IPv6 On-Link Redirects

[vangyzen]

Summary

Fix the handling of IPv6 On-Link Redirects

On receipt of a redirect message, install an interface route for the
redirected destination. On removal of the corresponding Neighbor Cache
entry, remove the interface route.

This requires changes in rtredirect_fib() to cope with an AF_LINK
address for the gateway and with the absence of RTF_GATEWAY.

This fixes the "Redirected On-Link" test cases in the Tahi IPv6 Ready Logo
Phase 2 test suite.

Unrelated to the above, fix a recursion on the radix node head lock
triggered by the Tahi Redirected to Alternate Router test cases.

When I first wrote this patch in October 2012, all Section 2
(Neighbor Discovery) test cases passed on 10-CURRENT, 9-STABLE,
and 8-STABLE. cem@ recently rebased the 10.x patch onto head and reported
that it passes Tahi. (Thanks!)

These other test cases also passed in 2012:

• the RTF_MODIFIED case, with IPv4 and IPv6 (using a

RTF_HOST|RTF_GATEWAY route for the destination)

• the redirected-to-self case, with IPv4 and IPv6

• a valid IPv4 redirect

All testing in 2012 was done with WITNESS and INVARIANTS.

Tested by: EMC / Isilon Storage Division via Conrad Meyer (cem) in 2015,

Mark Kelley <[e-mail]> in 2012,
TC Telkamp <[e-mail]> in 2012

PR: 152791
Reviewed by: bz
Approved by: kib (mentor)
MFC after: 1 month
Relnotes: yes
Sponsored by: Dell Inc.

Test Plan

See above.

[melifaro]

Looks fine to me.
The only thing that should be reconsidered is adding redirect to all fibs in system, but that could be changed in separate commit

[vangyzen]

Thanks for the review. Since the multi-fib handling is a separate and preexisting issue, I'll consider that for a followup commit.