neighbour: Drop read_lock_bh(&tbl->lock) in pneigh_dump_table().

neighbour: Drop read_lock_bh(&tbl->lock) in pneigh_dump_table().

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

Let's drop read_lock_bh(&tbl->lock) and use rcu_dereference()
to iterate tbl->phash_buckets[] in pneigh_dump_table()

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250716221221.442239-11-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>