net: remove the unnecessary strict_start_type in some policies

ct_policy and mpls_policy are parsed with nla_parse_nested(), which
does NL_VALIDATE_STRICT validation, strict_start_type is not needed
to set as it is actually trying to make some attributes parsed with
NL_VALIDATE_STRICT.

This patch is to remove it, and do the same on rtm_nh_policy which
is parsed by nlmsg_parse().

Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>