bpf: Fix mismatched RCU unlock flavour in bpf_out_neigh_v6

In the bpf_out_neigh_v6 function, rcu_read_lock() is used to begin an RCU
read-side critical section. However, when unlocking, one branch
incorrectly uses a different RCU unlock flavour rcu_read_unlock_bh()
instead of rcu_read_unlock(). This mismatch in RCU locking flavours can
lead to unexpected behavior and potential concurrency issues.

This possible bug was identified using a static analysis tool developed
by myself, specifically designed to detect RCU-related issues.

This patch corrects the mismatched unlock flavour by replacing the
incorrect rcu_read_unlock_bh() with the appropriate rcu_read_unlock(),
ensuring that the RCU critical section is properly exited. This change
prevents potential synchronization issues and aligns with proper RCU
usage patterns.

Fixes: 09eed11 ("neighbour: switch to standard rcu, instead of rcu_bh")
Signed-off-by: Jiawei Ye <jiawei.ye@foxmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/tencent_CFD3D1C3D68B45EA9F52D8EC76D2C4134306@qq.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

Author: pandayeah

net/core/filter.c

@@ -2249,7 +2249,7 @@ static int bpf_out_neigh_v6(struct net *net, struct sk_buff *skb,
 		rcu_read_unlock();
 		return ret;
 	}
-	rcu_read_unlock_bh();
+	rcu_read_unlock();
 	if (dst)
 		IP6_INC_STATS(net, ip6_dst_idev(dst), IPSTATS_MIB_OUTNOROUTES);
 out_drop: