nexthop: Fix attribute checking for groups

[ Upstream commit 84be69b869a5a496a6cfde9b3c29509207a1f1fa ]

For nexthop groups, attributes after NHA_GROUP_TYPE are invalid, but
nh_check_attr_group starts checking at NHA_GROUP. The group type defaults
to multipath and the NHA_GROUP_TYPE is currently optional so this has
slipped through so far. Fix the attribute checking to handle support of
new group types.

Fixes: 430a0491 ("nexthop: Add support for nexthop groups")
Signed-off-by: ASSOGBA Emery <assogba.emery@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>