selftests/net/ipsec: Add test for xfrm_spdattr_type_t

Set hthresh, dump it again and verify thresh.lbits && thresh.rbits.
They are passed as attributes of xfrm_spdattr_type_t, different from
other message attributes that use xfrm_attr_type_t.
Also, test attribute that is bigger than XFRMA_SPD_MAX, currently it
should be silently ignored.

Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>