Merge branch 'ipv6-ipv4-nexthop-align'

Ido Schimmel says:

====================
ipv6: Align nexthop behaviour with IPv4

This set tries to eliminate some differences between IPv4's and IPv6's
treatment of nexthops. These differences are most likely a side effect
of IPv6's data structures (specifically 'rt6_info') that incorporate
both the route and the nexthop and the late addition of ECMP support in
commit 51ebd3181572 ("ipv6: add support of equal cost multipath
(ECMP)").

IPv4 and IPv6 do not react the same to certain netdev events. For
example, upon carrier change affected IPv4 nexthops are marked using the
RTNH_F_LINKDOWN flag and the nexthop group is rebalanced accordingly.
IPv6 on the other hand, does nothing which forces us to perform a
carrier check during route lookup and dump. This makes it difficult to
introduce features such as non-equal-cost multipath that are built on
top of this set [1].

In addition, when a netdev is put administratively down IPv4 nexthops
are marked using the RTNH_F_DEAD flag, whereas IPv6 simply flushes all
the routes using these nexthops. To be consistent with IPv4, multipath
routes should only be flushed when all nexthops in the group are
considered dead.

The first 12 patches introduce non-functional changes that store the
RTNH_F_DEAD and RTNH_F_LINKDOWN flags in IPv6 routes based on netdev
events, in a similar fashion to IPv4. This allows us to remove the
carrier check performed during route lookup and dump.

The next three patches make sure we only flush a multipath route when
all of its nexthops are dead.

Last three patches add test cases for IPv4/IPv6 FIB. These verify that
both address families react similarly to netdev events.

Finally, this series also serves as a good first step towards David
Ahern's goal of treating nexthops as standalone objects [2], as it makes
the code more in line with IPv4 where the nexthop and the nexthop group
are separate objects from the route itself.

1. https://github.com/idosch/linux/tree/ipv6-nexthops
2. http://vger.kernel.org/netconf2017_files/nexthop-objects.pdf

Changes since RFC (feedback from David Ahern):
* Remove redundant declaration of rt6_ifdown() in patch 4 and adjust
comment referencing it accordingly
* Drop patch to flush multipath routes upon NETDEV_UNREGISTER. Reword
cover letter accordingly
* Use a temporary variable to make code more readable in patch 15
====================

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