neighbour: Annotate neigh_table.phash_buckets and pneigh_entry.next with __rcu.

The next patch will free pneigh_entry with call_rcu().

Then, we need to annotate neigh_table.phash_buckets[] and
pneigh_entry.next with __rcu.

To make the next patch cleaner, let's annotate the fields in advance.

Currently, all accesses to the fields are under the neigh table lock,
so rcu_dereference_protected() is used with 1 for now, but most of them
(except in pneigh_delete() and pneigh_ifdown_and_unlock()) will be
replaced with rcu_dereference() and rcu_dereference_check().

Note that pneigh_ifdown_and_unlock() changes pneigh_entry.next to a
local list, which is illegal because the RCU iterator could be moved
to another list.  This part will be fixed in the next patch.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250716221221.442239-7-kuniyu@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>