ipv6: remove one read_lock()/read_unlock() pair in rt6_check_neigh()

rt6_check_neigh() uses read_lock() to protect n->nud_state reading.

This seems overkill and causes false sharing.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/route.c

@@ -687,16 +687,16 @@ static enum rt6_nud_state rt6_check_neigh(const struct fib6_nh *fib6_nh)
 	neigh = __ipv6_neigh_lookup_noref(fib6_nh->fib_nh_dev,
 					  &fib6_nh->fib_nh_gw6);
 	if (neigh) {
-		read_lock(&neigh->lock);
-		if (neigh->nud_state & NUD_VALID)
+		u8 nud_state = READ_ONCE(neigh->nud_state);
+
+		if (nud_state & NUD_VALID)
 			ret = RT6_NUD_SUCCEED;
 #ifdef CONFIG_IPV6_ROUTER_PREF
-		else if (!(neigh->nud_state & NUD_FAILED))
+		else if (!(nud_state & NUD_FAILED))
 			ret = RT6_NUD_SUCCEED;
 		else
 			ret = RT6_NUD_FAIL_PROBE;
 #endif
-		read_unlock(&neigh->lock);
 	} else {
 		ret = IS_ENABLED(CONFIG_IPV6_ROUTER_PREF) ?
 		      RT6_NUD_SUCCEED : RT6_NUD_FAIL_DO_RR;