xfrm: Fix bucket count reported to userspace

sadhcnt is reported by `ip -s xfrm state count` as "buckets count", not the
hash mask.

Fixes: 28d8909b ("[XFRM]: Export SAD info.")
Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>