unify checkings for clean messages and fix indentation

Hi,

The following patches introduce a new function that check that the netlink messages received are clean. Actually some people performs such checkings, some don't and others simply do some half of them. I think that we must unify the behaviour of a netlink socket when it has to reply to malformed messages. Attached an indentation fix for netlink.h as well.

00indent-fix.patch:
fixes broken indentation in netlink.h
01process_skb.patch:
introduces the new function called netlink_check_skb that does
the sanity checkings for received messages.
02xfrm.patch:
the modification to make xfrm_user use such new function.
03rtnetlink.patch:
same thing for rtnetlink.

The 02 and 03 patches are straightforward conversions. If you are ok with this, I could post more patches to make other netlink sockets use this new function.

--
Pablo