nexthop: Only parse NHA_OP_FLAGS for dump messages that require it

The attribute is parsed in __nh_valid_dump_req() which is called by the
dump handlers of RTM_GETNEXTHOP and RTM_GETNEXTHOPBUCKET although it is
only used by the former and rejected by the policy of the latter.

Move the parsing to nh_valid_dump_req() which is only called by the dump
handler of RTM_GETNEXTHOP.

This is a preparation for a subsequent patch.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240311162307.545385-3-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/nexthop.c

@@ -3398,11 +3398,6 @@ static int __nh_valid_dump_req(const struct nlmsghdr *nlh, struct nlattr **tb,
 		return -EINVAL;
 	}
 
-	if (tb[NHA_OP_FLAGS])
-		filter->op_flags = nla_get_u32(tb[NHA_OP_FLAGS]);
-	else
-		filter->op_flags = 0;
-
 	return 0;
 }
 
@@ -3418,6 +3413,11 @@ static int nh_valid_dump_req(const struct nlmsghdr *nlh,
 	if (err < 0)
 		return err;
 
+	if (tb[NHA_OP_FLAGS])
+		filter->op_flags = nla_get_u32(tb[NHA_OP_FLAGS]);
+	else
+		filter->op_flags = 0;
+
 	return __nh_valid_dump_req(nlh, tb, filter, cb->extack);
 }