[XFRM] netlink: Use nlmsg_new() and type-safe size calculation helpers

Moves all complex message size calculation into own inlined helper
functions and makes use of the type-safe netlink interface.

Using nlmsg_new() simplifies the calculation itself as it takes care
of the netlink header length by itself.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>