neighbour: Convert RTM_SETNEIGHTBL to RCU.

neightbl_set() fetches neigh_tables[] and updates attributes under
write_lock_bh(&tbl->lock), so RTNL is not needed.

neigh_table_clear() synchronises RCU only, and rcu_dereference_rtnl()
protects nothing here.

If we released RCU after fetching neigh_tables[], there would be no
synchronisation to block neigh_table_clear() further, so RCU is held
until the end of the function.

Another option would be to protect neigh_tables[] user with SRCU
and add synchronize_srcu() in neigh_table_clear().

But, holding RCU should be fine as we hold write_lock_bh() for the
rest of neightbl_set() anyway.

Let's perform RTM_SETNEIGHTBL under RCU and drop RTNL.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251022054004.2514876-5-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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