net, neigh: Use NLA_POLICY_MASK helper for NDA_FLAGS_EXT attribute

Instead of open-coding a check for invalid bits in NTF_EXT_MASK, we can just
use the NLA_POLICY_MASK() helper instead, and simplify NDA_FLAGS_EXT sanity
check this way.
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/neighbour.c

@@ -1834,7 +1834,7 @@ const struct nla_policy nda_policy[NDA_MAX+1] = {
 	[NDA_MASTER]		= { .type = NLA_U32 },
 	[NDA_PROTOCOL]		= { .type = NLA_U8 },
 	[NDA_NH_ID]		= { .type = NLA_U32 },
-	[NDA_FLAGS_EXT]		= { .type = NLA_U32 },
+	[NDA_FLAGS_EXT]		= NLA_POLICY_MASK(NLA_U32, NTF_EXT_MASK),
 	[NDA_FDB_EXT_ATTRS]	= { .type = NLA_NESTED },
 };
 
@@ -1936,10 +1936,6 @@ static int neigh_add(struct sk_buff *skb, struct nlmsghdr *nlh,
 	if (tb[NDA_FLAGS_EXT]) {
 		u32 ext = nla_get_u32(tb[NDA_FLAGS_EXT]);
 
-		if (ext & ~NTF_EXT_MASK) {
-			NL_SET_ERR_MSG(extack, "Invalid extended flags");
-			goto out;
-		}
 		BUILD_BUG_ON(sizeof(neigh->flags) * BITS_PER_BYTE <
 			     (sizeof(ndm->ndm_flags) * BITS_PER_BYTE +
 			      hweight32(NTF_EXT_MASK)));