ipv6: Factorise ip6_route_multipath_add().

We will get rid of RTNL from RTM_NEWROUTE and SIOCADDRT and rely
on RCU to guarantee dev and nexthop lifetime.

Then, the RCU section will start before ip6_route_info_create_nh()
in ip6_route_multipath_add(), but ip6_route_info_create() is called
in the same loop and will sleep.

Let's split the loop into ip6_route_mpath_info_create() and
ip6_route_mpath_info_create_nh().

Note that ip6_route_info_append() is now integrated into
ip6_route_mpath_info_create_nh() because we need to call different
free functions for nexthops that passed ip6_route_info_create_nh().

In case of failure, the remaining nexthops that ip6_route_info_create_nh()
has not been called for will be freed by ip6_route_mpath_info_cleanup().

OTOH, if a nexthop passes ip6_route_info_create_nh(), it will be linked
to a local temporary list, which will be spliced back to rt6_nh_list.
In case of failure, these nexthops will be released by fib6_info_release()
in ip6_route_multipath_add().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418000443.43734-12-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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