neighbour: Use rcu_dereference() in pneigh_get_{first,next}().

Now pneigh_entry is guaranteed to be alive during the
RCU critical section even without holding tbl->lock.

Let's use rcu_dereference() in pneigh_get_{first,next}().

Note that neigh_seq_start() still holds tbl->lock for the
normal neighbour entry.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250716221221.442239-12-kuniyu@google.com


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