netfilter: ipset: Improve skbinfo get/init helpers

Use struct ip_set_skbinfo in struct ip_set_ext instead of open
coded fields and assign structure members in get/init helpers
instead of copying members one by one. Explicitly note that
struct ip_set_skbinfo must be padded to prevent non-aligned
access in the extension blob.

Ported from a patch proposed by Sergey Popovich <popovich_sergei@mail.ua>.

Suggested-by: Sergey Popovich <popovich_sergei@mail.ua>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>