ipv6: Get rid of RTNL for SIOCDELRT and RTM_DELROUTE.

Basically, removing an IPv6 route does not require RTNL because
the IPv6 routing tables are protected by per table lock.

inet6_rtm_delroute() calls nexthop_find_by_id() to check if the
nexthop specified by RTA_NH_ID exists.  nexthop uses rbtree and
the top-down walk can be safely performed under RCU.

ip6_route_del() already relies on RCU and the table lock, but we
need to extend the RCU critical section a bit more to cover
__ip6_del_rt().  For example, nexthop_for_each_fib6_nh() and
inet6_rt_notify() needs RCU.

Let's call nexthop_find_by_id() and __ip6_del_rt() under RCU and
get rid of RTNL from inet6_rtm_delroute() and SIOCDELRT.

Even if the nexthop is removed after rcu_read_unlock() in
inet6_rtm_delroute(), __remove_nexthop_fib() cleans up the routes
tied to the nexthop, and ip6_route_del() returns -ESRCH.  So the
request was at least valid as of nexthop_find_by_id(), and it's just
a matter of timing.

Note that we need to pass false to lwtunnel_valid_encap_type_attr().
The following patches also use the newroute bool.

Note also that fib6_get_table() does not require RCU because once
allocated fib6_table is not freed until netns dismantle.  I will
post a follow-up series to convert such callers to RCU-lockless
version.  [0]

Link: https://lore.kernel.org/netdev/20250417174557.65721-1-kuniyu@amazon.com/ #[0]
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418000443.43734-3-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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